1 Introduction

With increasing awareness of environmental protection, more countries are committing to achieving net-zero emissions in the future[1]. According to the International Energy Agency, the target is to reach this goal by 2050, as illustrated in Figure 1a. Achieving this target requires reducing carbon emissions from the industrial, power, and transportation sectors, which together represent 77% of total emissions. (Figure 1b) [1]. The industrial sector is the largest contributor, consuming over 50% of the world's electricity [2]. Thus, reducing carbon emissions and lowering energy consumption will be central to future developments in the manufacturing industry [3]. Traditional manufacturing often relies on cutting fluids for cooling, lubrication, and chip removal, using substantial amounts of these fluids [4,5,6]. However, their extensive use brings several issues:

  1. (1)

    Cutting fluids usually contain base oils, which may be mineral oil, synthetic oil, or vegetable oil [7]. Approximately 85% of cutting fluids worldwide are mineral oil-based [7,8,9]. Over 4 million tons are consumed annually, and their discharge into the environment causes pollution issues and impacts limited mineral and freshwater resources [10].

  2. (2)

    Traditional metalworking fluids produce significant amounts of oil mist and PM2.5 around high-speed rotating tools and high-temperature cutting areas [11]. Bacteria that grow in reused cutting fluids can spread into the air with the oil mist. Workers inhaling this mist risk developing respiratory diseases, including mild respiratory issues, asthma, and even cancer [12, 13].

Figure 1
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(a) Green development planning diagram [1], (b) Industry cost breakdown, (c) Detailed manufacturing cost distribution diagram

As environmental regulations become stricter, the costs associated with cutting fluids continue to rise. Disposal costs can represent 18% to 21% of total expenses, which is 3 to 5 times the cost of tools [5, 14,15,16,17,18,19] (Figure 1c). The total procurement, maintenance, and disposal costs of traditional metalworking fluids (MWFs) are double that of machining, comprising 7% to 17% of total manufacturing costs [20,21,22,23].

Traditional flood lubrication and cooling pose challenges to sustainable development in the manufacturing industry [24,25,26,27,28]. Achieving sustainability requires technological innovation [29,30,31]. In response, manufacturing powerhouses have implemented stringent laws and standards, such as carbon emission standards (ISO 14067-2018) and the "Clean Production Promotion Law." Researchers are exploring green machining methods, focusing on concepts like dry cutting [32,33,34,35,36], near-dry cutting [37,38,39,40], and cryogenic machining [41,42,43,44,45,46].

Grinding processing is notable for its high precision, versatility, and excellent surface quality [47,48,49,50,51,52]. It plays a vital role in ensuring the final precision and surface quality of many components [49, 53]. In fact, over 60% of machining processes for aerospace materials involve grinding [54, 55]. These materials are challenging to machine due to their low thermal conductivity, high specific grinding energy, and high fracture toughness, which increases their susceptibility to thermal damage during high-temperature grinding [56,57,58]. The negative rake angle of grinding wheel abrasives results in significantly higher energy consumption, heat generation, and cutting fluid use compared to other machining methods [59, 60]. Therefore, the sustainability of grinding processes needs enhancement. Researchers have proposed various green grinding methods, including dry grinding [61,62,63], solid lubrication grinding [64, 65], minimum quantity lubrication(MQL) grinding [66,67,68], and cryogenic grinding [69, 70].

Although existing articles have reviewed these grinding processes from various perspectives, there remains a lack of visual analysis regarding their development trends, key authors, and research hotspots in green grinding [71, 72]. This article examines the current state of key research areas in green grinding and utilizes bibliometric analysis to identify emerging trends. We provide a visual analysis of annual publication volume, countries, institutions, authors, source journals, keyword co-occurrence, and co-citation distributions from the Web of Science for the years 2008–2023. Based on this bibliometric analysis, we also conduct a comprehensive review of the development of green grinding.

When faced with a vast number of articles, it is easy to feel overwhelmed by the data if appropriate reading and analysis strategies are not implemented. Bibliometric analysis is a scientific and quantitative research method [73] for analyzing articles. By visualizing articles on green grinding, one can quickly grasp development trends, identify reputable authors and research institutions, and determine credible research topics. This process ultimately accelerates advancements in scientific research.

The overall framework of the paper is illustrated in Figure 2. This study focused on relevant keywords such as dry grinding, cryogenic grinding, environmentally friendly grinding, and green grinding, with a publication timeframe from 2008 to 2023. After manual selection, 738 articles were identified. Bibliometric methods were then applied to analyze these articles, revealing current hotspots and future trends. A comprehensive review of current environmentally friendly grinding methods was also conducted.

Figure 2
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Paper framework diagram

2 Data and Visualization

The Web of Science (WOS) is a source of citation information for bibliometric investigations. It contains more scientific publications than Scopus, Derwent, the China National Knowledge Infrastructure (CNKI), and the Chinese Social Sciences Citation Index (CSSCI). Furthermore, WOS provides general statistical data essential for bibliometric software. It is the most commonly used database in bibliometric research. The process of data retrieval and collection is depicted in Figure 2. We searched for phrases including "green grinding," "MQL," "dry grinding," "solid lubrication grinding," and "cryogenic cooling grinding," limiting publications to 2008–2023. The retrieved papers were saved as "plain text," and citation records were exported as "full text." The final file was named "green grinding.TXT."

VOSviewer [74], Bibliometrix [75], and Origin are software tools utilized for analyzing the statistical data collected from the WOS Core Collection. These tools facilitate data visualization. Co-authorship, co-occurrence, and co-citation are key indicators used in bibliometric analysis. Co-authorship analysis examines the connections between institutions, authors, and countries. Co-occurrence quantitatively assesses the relationships among various elements. Co-citation statistical analysis highlights the significance of cited articles. For this paper, we collected the following basic data: authors, institutions, countries/regions, journals, keywords, and references.

3 Bibliometric Analysis

By analyzing data retrieved from WOS, several insights emerge. This includes global trends in researcher numbers and publication rates, the major contributing institutions and countries, and, through author tracing, both individual and team contributions to the field.

3.1 Global Publication Output Trends

A total of 738 articles on green grinding were identified in WOS, authored by researchers across 59 countries. Figure 3 shows the global trends in sustainable grinding technology publications from 2008 to 2023. After 2008, there was a steady rise in publications related to green grinding, with the annual publication count increasing from 10 in 2008 to 112 in 2023, reflecting an 8.34% annual growth rate. A notable uptick in publications occurred after 2015.

Figure 3
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Global publication trends on environmentally friendly grinding from 2008 to 2023

3.2 The Distribution of Countries/Regions Involved in this Research indicates the Following

Articles on this topic have been published across 115 countries/regions. Table 1 lists the top ten countries/regions for eco-friendly grinding articles, with China leading at an annual growth rate, followed by India. China accounts for 45.38% of publications, with India contributing 17.44%. Papers published by China also hold the highest citation count (13300), far exceeding India's (3430), underlining China's strong influence in this area. SCP represents the number of papers co-authored by authors from the same country, while MCP refers to those co-authored with authors from other countries. The MCP ratio, a measure of global collaboration, shows Malaysia, Spain, Germany, and Poland at the forefront of international partnerships. Although Germany's publication volume is lower, its high average citation per paper and MCP% highlight its collaborative and impactful research.

Table 1 Top ten high-productivity countries/regions in eco-friendly grinding
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Figure 4a illustrates China's collaborations, with India and the United States as key partners. Figure 4b, generated with VOSviewer, includes 41 countries with at least five publications, while where circle size indicates publication volume and lines indicate collaboration strength.

Figure 4
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(a) Visualization map of international collaboration among countries/regions; (b) Citation network visualization map generated using VOSviewer for different countries/regions. The number of lines reflects the strength of citations

3.3 Contributions of Various Institutions and Authors

A total of 708 research institutions have published on sustainable grinding processes. Table 2 lists the top ten institutions based on publication volume, with most concentrated in China, India, and Brazil. São Paulo State University, Qingdao University of Technology, and the Indian Institute of Technology lead in publication output. Figure 5 shows collaboration intensity among institutions, with more connections indicating closer partnerships.

Table 2 Top 10 high-productivity institutions ranked by number of published papers
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Figure 5
Figure 5
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Institutional collaboration diagram

3.4 Author Analysis

The study included 2055 researchers. Table 3 presents the ten most productive authors in green grinding technology.

Table 3 Top 10 high-productivity authors ranked by number of published papers
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Figure 6 shows the author collaboration network, revealing two main research teams and related researchers who have made significant contributions to green grinding.

Figure 6
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Author collaboration diagram

There is a degree of collaboration among these authors, represented by lines connecting circles; more lines indicate closer collaboration. Each circle represents an individual researcher, with larger circles indicating higher publication counts. Notably, EC Bianchi from Brazil has published the most papers (81), whereas CH Li from China, with 80 papers, has the highest H-index (48) and G-index (80). Additionally, YB Zhang from China has an H-index of 46 and a G-index of 80. High H-index and G-index values suggest these authors have considerable influence in the field.

3.5 Distribution of Source Journals and the Top 10 High-cited Articles

Research on green grinding has been published in 175 different journals, with the top ten accounting for over 50% of total publications. The International Journal of Advanced Manufacturing Technology has the highest citation and publication volume, contributing 22.9% of the total (Table 4). The International Journal of Machine Tools & Manufacture has the highest average citation count and impact factor (IF), followed by the Journal of Cleaner Production. These results indicate that these journals have played a major role in advancing the topic. Among the top ten journals, 30% are classified as Q1, and 40% as Q2.

Table 4 Top Ten journals by number of published articles
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Table 5 lists the ten most highly cited articles on green grinding, all of which focus on MQL grinding technology. Published within the last ten years in Q1 journals, these articles highlight the growing recognition of MQL as an emerging technology.

Table 5 Top 10 high-cited articles
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4 Discussion

Figure 7 illustrates the continuous evolution of green grinding technology. Early techniques like dry grinding [61,62,63, 85,86,87] and solid lubrication [64, 88,89,90,91,92] gained popularity in the 1990s but were limited by excessive heat generation, which often damages workpieces, restricting their industrial use. Although cryogenic grinding was also proposed early in Refs. [93,94,95,96,97,98], its application is constrained by the complexities of cryogenic media storage, transport, and installation. In contrast, MQL and its related technologies have rapidly gained traction in both industry and academia due to their ease of installation and operation. By 2024, publications on MQL comprise 70% of the literature on eco-friendly grinding technologies (Figure 7), with publication rates rising annually. Moreover, MQL is increasingly integrated with other technologies, marking a shift toward a new technological paradigm.

Figure 7
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Development of eco-friendly grinding

4.1 Global Trends on Green Grinding Research

Figure 3 shows the growth in green grinding publications globally from 2008 to 2023. After 2015, publication rates for sustainable grinding technologies rose significantly, with the highest growth observed in the past five years. Given the environmental standards in various countries, it is likely that green grinding research will continue to expand. Surveys show that China and India lead in publication volume, with China also producing most high-citation papers, likely due to its large manufacturing sector.

4.2 Hotspots and Emerging Frontiers in Green Grinding

Using VOSviewer, a keyword analysis of all articles was conducted, with results shown in Figure 8. The keyword "MQL" appears most frequently. Aside from terms like surface roughness, temperature, and grinding force, the most common keywords include "nano-fluids" and "plant oil-based" (Table 6).

Figure 8
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Keyword Co-occurrence

Table 6 Keyword frequency
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Table 6 lists the most frequently used keywords, with the top ten highly cited articles primarily focusing on MQL, particularly in nanoparticle and plant oil selection and performance.

4.2.1 MQL Grinding

MQL grinding, a near-dry machining process [79, 99,100,101], utilizes MQL equipment as shown in Figure 9a. This method applies a small amount of liquid lubricant (typically less than 100 mL/h) atomized into micron-sized droplets by pressurized gas, which is then directed to the cutting zone [102,103,104]. Compared to traditional flood cooling, MQL reduces lubricant use by over 10000 times [105]. Sometimes referred to as Small Quantity Lubrication (SQL) [106, 107], MQL combines the benefits of both dry machining and flood cooling, with performance that falls between the two, hence the term "near-dry cutting" [108,109,110,111].

Figure 9
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Schematic of MQL grinding

As shown in Figure 9b, micro-droplets provide effective lubrication between the tool and workpiece, enhancing the surface integrity of both the part and the grinding wheel [112]. High-pressure airflow breaks the air boundary layer around the grinding zone, lowering temperatures and removing chips [113]. Sharma et al. [79] found that MQL processing achieves better surface quality than dry machining. Furthermore, MQL's cost-effectiveness, environmental benefits, and health advantages have drawn increasing research interest [67, 114,115,116].

4.2.2 Biolubricants

Compared to petroleum-based lubricants, bio-lubricants offer advantages in environmental sustainability, presenting a promising alternative. Factors influencing bio-lubricants performance include the type of bio-lubricant, fatty acids, carbon chain length, and viscosity.

4.2.2.1 Types and Molecular Structures of Biolubricants

Kelly et al. [117] first proposed vegetable oils as machining coolants in 2002. With natural, eco-friendly, and biodegradable properties, biolubricants align with the principles of Refs. [118,119,120,121]. Vegetable oils, with low volatility, good viscosity index, and excellent lubrication, are environmentally promising [122, 123]. Commonly used in MQL are oils like soybean, canola, sunflower, sesame, castor, olive, corn, and peanut oils, along with synthetic esters (Figure 10a) [124]. Composed mainly of triglycerides (Figure 10b), they also contain small amounts of free fatty acids, phospholipids, and glycerides [125, 126].

Figure 10
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(a) Common vegetable oils and their components, (b) Effect of fatty acid components on performance [127]

4.2.2.2 Fatty Acids

A triglyceride molecule contains three fatty acids, whose types vary with the oil source [128] (Figure 11b). Fatty acids significantly influence the grinding performance of vegetable oils [127], comprising a carbon chain and polar groups (e.g., hydroxyl, carboxyl) [129]. The type and concentration of fatty acids determine physicochemical properties, which affect cooling and lubrication during grinding [130]. Polar groups in bio-lubricants readily adhere to metal surfaces [131], forming a lubricating film [35, 132, 133] (Figure 11a). Fatty acids can also undergo saponification with metal surfaces at high temperatures, forming a monomolecular layer that reduces grinding forces and extends tool life [134].

Figure 11
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(a) Lubrication mechanisms of biolubricants, (b) Structure of fatty acids, (c) Lubrication mechanisms of saturated and unsaturated fatty acids

Fatty acids are classified into saturated and unsaturated fatty acids [135, 136]. Most vegetable oils have a high content of unsaturated fatty acids [137, 138]. Unsaturated fatty acids exhibit better viscosity and surface tension, which supports both lubrication and cooling [121]. However, they typically contain C=C double bonds, which are susceptible to oxidation, causing biolubricants to degrade and leading to the breakdown of the physical adsorption film [139]. To improve the chemical stability of vegetable oils, which is essential for performance, antioxidants are commonly added [140,141,142]. Another approach involves chemically modifying the vegetable oils. By adjusting the degree of unsaturation, carbon chain length, and branching of fatty acids through reactions involving the carboxyl groups and carbon chains of unsaturated fatty acids, the thermal oxidation stability of vegetable oils can be enhanced. Common modification methods include hydrogenation [143, 144], epoxidation [144,145,146], and transesterification [144, 147].

Additionally, saturated fatty acids are gaining attention for their higher viscosity and superior lubrication properties [148]. Due to the absence of C=C double bonds, saturated fatty acid molecules tend to arrange into elongated, straight chains, forming dense lubricating films that enhance lubrication [149] (Figure 11c). Zhang et al. reported that palm oil-based nanofluids provided the best lubrication performance when using soybean oil, palm oil, and canola oil for grinding 45 steel [81]. Padmini et al. examined MoS₂ nanofluids based on coconut, sesame, and canola oils for processing AISI 1040 steel. Coconut oil-based nanofluids achieved the best processing performance, with tool wear 31.58% lower than with canola oil [150]. Coconut oil has a saturated fatty acid content of approximately 90% [151]. Yin et al. [134] systematically investigated the lubrication mechanisms of various vegetable oils with different physicochemical properties (cottonseed oil, palm oil, castor oil, soybean oil, and peanut oil). Palm oil, with its high saturated fatty acid content and viscosity, mainly composed of palmitic and stearic acids, formed the most effective dispersive and lubricating film at the wheel-workpiece interface.

4.2.2.3 Carbon Chain Length

Fatty acids with longer carbon chains form stronger lubricating films, providing better lubrication than those with shorter chains due to increased molecular cohesion [152]. For instance, canola oil (C22) has a longer carbon chain than soybean oil (C18), resulting in improved lubrication performance [153]. However, for saturated fatty acids, lubrication benefits plateau once the carbon chain length reaches 16 atoms [154]. Therefore, saturated fatty acids with linear molecular chains and no C=C double bonds form denser films, enhancing lubrication strength.

4.2.2.4 Viscosity

Viscosity is a critical factor affecting the cooling and lubricating performance of biolubricants [80]. High-viscosity lubricants, whereas offering better lubrication properties, may hinder flowability, preventing effective lubrication and debris removal in the grinding zone. Nonetheless, these lubricants reduce friction between the abrasive and workpiece, lowering grinding temperatures. Figure 12a shows viscosities of common vegetable oils.

Figure 12
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(a) Viscosity of common plant oils [155], (b) Relationship between viscosity and wetting properties

Higher fluid viscosity results in a thicker viscous sublayer, which increases surface tension and contact angle of droplets, reducing wettability and heat dissipation on the grinding surface (Figure 12b).

Although castor oil has excellent lubrication properties, its high viscosity limits its use [80]. Researchers have blended different types of vegetable oils to combine their benefits and enhance the overall performance of the lubricant [156, 157]. Guo et al. [80] used castor oil as the base oil and mixed it in a 1:1 ratio with six other plant oils (soybean, maize, peanut, sunflower, palm, and rapeseed oils) to create a blended oil. When tested for grinding nickel alloy GH4169, the results indicated that the blended oils outperformed castor oil. Among the blends, soybean/castor oil was the most effective, with tangential and normal grinding forces decreasing by 27.03% and 23.15%, respectively, compared to the base oil.

4.2.3 Nanobiological Enhanced Lubricant Minimum Quantity Lubrication Grinding

Zhang et al. [158] found that burn marks appeared on the workpiece surface during MQL grinding of nickel alloys. Wu et al. [159] added nanoparticles to biolubricants to enhance lubrication and grinding performance, a method known as nano-biological enhanced lubricant minimum quantity lubrication (NMQL) [121, 142]. Different nanoparticles have varying mechanisms for reducing friction and conducting heat [160, 161].

The preparation of nanofluids primarily involves one-step and two-step methods [162, 163]. The one-step method directly disperses nanoparticles into the base fluid during preparation [164, 165]. This method has higher production costs and is currently unsuitable for large-scale applications [166, 167]. The two-step method involves dispersing nanoparticles into the base liquid using physical or chemical methods, such as ultrasonic dispersion, mechanical stirring, high-pressure homogenization, and the addition of dispersants. Although the two-step method may lead to oxidation of nanoparticles and potential aggregation, it is simple to operate and cost-effective, making it widely used.

The factors influencing the lubrication performance of nanoparticles primarily include the type of nanoparticles, their shape, and their structure.

4.2.3.1 Types of Nanoparticles

Nanoparticles include both metallic and non-metallic types. Among non-metallic nanoparticles, carbon-based nanoparticles have unique self-lubricating properties, making them widely studied in both fluid and solid lubrication fields [161]. Commonly used nanoparticles include spherical, tubular, and layered types, as shown in Figure 13.

Figure 13
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Common nanoparticles

Nanoparticles with varying chemical properties and structures exhibit significant differences in cooling and lubrication performance [168,169,170]. Generally, spherical and layered nanoparticles provide higher lubrication performance, whereas tubular nanoparticles have micro-abrasive capabilities [171].

Chemically, nanoparticles can be classified as metallic, oxide, carbon, and sulfide nanoparticles [60]. Metallic nanoparticles have good thermal conductivity, with copper nanoparticles even possessing self-repairing properties [163]. However, they lack cost advantages for industrial applications. Oxide nanoparticles are commonly used, and when employing the two-step method to prepare nanofluids, oxide nanoparticles help prevent oxidation. Carbon nanoparticles offer high thermal conductivity, stability, and wear resistance [172]. Their chemical inertness and large surface area have made them a research focus in recent years. Sulfide nanoparticles, with weak sulfur-sulfur bonds between layers, are prone to delamination, which provides lubricating effects.

As shown in Figure 14a, the anti-wear and friction reduction mechanisms of nanoparticles at the friction contact interface are summarized as follows:

Figure 14
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(a) Mechanisms of friction reduction by nanoparticles, (b) Effects of different nanoparticles on the friction coefficient [178], (c) SEM images of grinding debris under different lubrication conditions [173]

  1. (1)

    Bearing Effect of Spherical Nanoparticles: Spherical nanoparticles, especially onion-like structures, act as micro-bearings in Ref. [174]. These particles gradually shed layers during lubrication [175], converting sliding friction between contact surfaces into rolling or sliding friction within the nanoparticles, significantly lowering the friction coefficient [176].

  2. (2)

    Formation of Lubricating Films: Nanoparticles can form smooth, dense lubricating films on tool-workpiece surfaces. Layered nanoparticles, in particular, are flexible and ductile [177]. Thin layers shed during sliding and adsorb onto the machining surface, helping to reduce friction [99].

  3. (3)

    Surface Repair: Nanoparticles can fill micro-cracks and deposit them onto friction surfaces, effectively repairing micro-damage and contributing to a smoother machining surface [178].

  4. (4)

    Polishing by Hard Nanoparticles: Hard nanoparticles provide precision polishing, which reduces surface roughness while, lowering contact stress, and increasing the load-carrying capacity of the lubricant [179].

Wang et al. [173] compared the sliding friction coefficients of six nanobiolubricants for grinding nickel-based alloys, as well as conventional flood lubrication and pure palm oil MQL. NMQL demonstrated the best lubrication performance (Figure 14b). SEM images show that nanofluid MQL produced smoother, longer, and thinner chips (Figure 14c).

Kumar et al. [180] examined the effects of adding Al2O3, hexagonal boron nitride (HBN), and WS2 to the grinding fluid for Ti-6Al-4V. Al2O3 nanobiolubricants exhibited superior anti-wear properties due to their hard-phase characteristics (HR = 2700–3000), providing a solid support phase on the tool-workpiece surface that reduces friction while offering a polishing effect. With a high melting point of up to 2050 °C, Al2O3 is also effective at elevated temperatures [181]. Mao et al. [182] found that adding Al2O3 nanoparticles to the base fluid reduced the friction coefficient by 34.2% during grinding of AISI 52100 steel. Jia et al. observed that although MoS2 nanobiolubricants had a slightly higher friction coefficient than Al2O3 in grinding ductile iron, their layered structure provided better lubrication, as interlayer bonds readily break, facilitating sliding and reducing friction.

Many studies have confirmed that adding nanoparticles improves the heat transfer rate of mixed fluids compared to pure base fluids (Figure 15c) [149, 183,184,185,186,187]. Enhanced heat transfer is critical for better machining performance and tool longevity. The mechanism of heat transfer in nanobiolubricants is illustrated in Figure 15a [188]. When nanoparticles are introduced, Brownian motion increases contact between nanoparticles and the workpiece, allowing them to absorb and dissipate heat. A layer of vegetable oil molecules forms around each nanoparticle, transferring excess heat away from the contact area as these layers interact.

Figure 15
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(a) Heat transfer principles, (b) contact angle for six different nanoparticle [99], (c) Grinding temperature under different lubrication conditions [149]

In MQL grinding, lower surface tension in nanobiolubricants improves droplet break-up and atomization, enhancing lubrication and cooling of the grinding wheel and workpiece [189]. Li et al. [99] compared the contact angles of different nanobiolubricants on nickel-based alloy surfaces (Figure 15b). ZrO2 had the smallest contact angle, indicating superior wettability and heat transfer performance. However, CNTs showed the lowest grinding temperatures due to their higher convective heat transfer coefficient. This suggests that optimal heat transfer performance results from a combination of multiple factors.

4.2.3.2 Hybrid Nanobiolubricants

Combining different types of nanoparticles has become a cost-effective way to enhance both cooling and lubrication performance [136, 179, 190]. Extensive research has explored hybrid nanobiolubricants. For example, Zhang et al. [77] combined MoS2 nanoparticles with carbon nanotubes (CNTs) to study their synergistic lubrication effects. As shown in Figure 16a, the adsorption of MoS2 onto CNTs improves the properties of CNT nanoparticles, which tend to cluster. The high strength and hardness of CNTs allow them to withstand high loads, forming durable films that prevent MoS2 from failing at elevated temperatures and pressures. In their study, increasing the proportion of CNTs in the MQL grinding fluid led to more CNT agglomeration due to van der Waals forces, which slightly reduced lubrication efficiency. Nonetheless, MoS2/CNT (2:1) hybrid nanobiolubricants reduced friction by 13.1% and improved surface roughness by 13% during nickel alloy grinding compared to pure MoS2 or CNT lubricants.

Figure 16
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(a) Principle of friction reduction by mixed nanoparticles, (b) The surface roughness in grinding with mixed nanoparticles [162]

Zhang et al. [158] investigated the grinding performance of Al2O3/SiC hybrid nanobiolubricants on nickel-based alloys. Al2O3 nanoparticles, known for strong adsorption, attach to SiC nanoparticle surfaces, enhancing dispersion and stability in the lubricant (Figure 16b).

4.3 Other Technology Combined with MQL

Although various MQL enhancement techniques have been proposed to improve machining performance, they generally focus on extending lubricant application without changing its lubrication mechanism. Although NMQL can improve machining conditions, it does not fundamentally alter the lubricant's underlying lubrication mechanism.

Under extreme boundary conditions, such as high temperatures and pressures, biolubricants risk failure. To address these, external energy fields, such as low-temperature fields, electric fields, magnetic fields, and ultrasound, have been applied to improve grinding performance.

4.3.1 Cryogenic-MQL

Although NMQL reduces cooling rates by up to 20% compared to standard MQL [191], making it suitable for easily machined materials like aluminum alloys [192], thermal damage remains a concern when grinding difficult-to-machine materials [193]. MQL reduces frictional heat between the grinding wheel and workpiece, limiting [194]. Deep cooling technology can inhibit thermal softening and refine grain structure, increasing material hardness [195,196,197,198].

Cooling minimum quantity lubrication (CMQL) combines low-temperature and MQL technologies [199,200,201]. This approach lowers grinding temperatures while maintaining effective lubrication, improving surface quality and extending tool life [202,203,204]. Typical coolants include liquid CO2, N2, and low-temperature air, each with a specific cooling range. Liquid nitrogen provides the lowest temperature (−80°C to −196°C), followed by liquid carbon dioxide (− 25 °C to − 78.5 °C), and low-temperature air (0°C to – 50 °C), as shown in Figure 17b.

Figure 17
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(a) LCO2+MQL grinding system [20], (b) Physical states of different cooling media at various temperatures

Sanchez et al. [20] proposed an LCO₂+MQL system that enhances MQL heat dissipation. The MQL nozzle sprays a small amount of oil onto the grinding wheel, followed by an LCO₂ nozzle positioned below it, aligned with the wheel's rotation. The LCO2 spray cools the grinding area and solidifies the oil on abrasive grains, forming a durable film that penetrates the wheel-workpiece interface, providing lubrication. As illustrated in Figure 17a, the LCO2 cooling reduces surface hardening and cutting forces compared to liquid nitrogen, which tends to over-harden the material [205]. Proper nozzle sizing ensures the low-temperature air reaches the grinding zone unobstructed by wheel airflow [67].

High temperatures, speeds, and pressures in the cutting zone can reduce lubricant viscosity [207, 208], thinning and rupturing the oil film, potentially exposing the workpiece. When the temperature exceeds a critical threshold, thermal degradation can cause the film to fail [209]. As shown in Figure 18a, low temperatures maintain lubricant effectiveness and improve load capacity. CMQL prevents high-temperature film failure (Figure 18b), though low-temperature micro-droplets increase surface tension and contact angle [210], reducing penetration and thus lubrication effectiveness.

Figure 18
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(a) The relationship between temperature and viscosity, (b) Effect of temperature on droplets and polar groups [206]

Zhang et al. [211] compared the effects of NMQL, Cryogenic Air (CA), and CA+NMQL on grinding temperature during Ti-6Al-4V grinding (Figure 19a). The addition of nanoparticles in CNMQL resulted in the lowest grinding temperatures.

Figure 19
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(a) Grinding temperature under different conditions [211], (b) Specific grinding energy under different conditions [212], (c) Variation of surface roughness with grinding depth under different conditions [83], (d) Relationship between material removal rate and energy consumption under different lubrication conditions [212]

Zhang et al. [83] also found that CA+NMQL yielded the lowest specific grinding energy, with specific energies for CA and NMQL 33.1% and 12.3% higher, respectively, than CA+NMQL (Figure 19c). Garcia et al. [212] observed that the flow rate Q of LCO2 significantly impacts specific grinding energy (Figure 19b); at Q=15 ml/min, specific energy decreased sharply, whereas at Q=3 ml/min, insufficient oil film lubrication led to quicker wheel dulling and reduced cutting effectiveness (Figure 19d). Zhang et al. [213] compared dry grinding, flood cooling, CA+MQL, and CA+NMQL in grinding GCr15, finding that CA+MQL achieved the best surface morphology.

4.3.2 Electrostatic Atomization for Minimum Quantity Lubrication

In MQL grinding, whereas increasing gas pressure refines droplet size and enhances penetration, it also generates substantial oil mist particles, increasing the risk of droplet dispersion. Oil mist dispersion in MQL raises environmental and health while concerns and reduces lubricant efficiency [214,215,216]. Additionally, bio-lubricants with high viscosity and surface tension hinder heat transfer and diffusion into the cutting zone, limiting their use.

Electrostatic atomization uses a high-voltage electrostatic field to atomize and charge liquids. The charged droplets pass through a corona zone, acquiring an electric charge through contact or non-contact methods. This charge decreases the droplets' surface tension, triggering further fragmentation and forming clusters of charged droplets. Electrostatic atomization thus reduces droplet size, improves droplet distribution uniformity, and enhances adhesion, deposition efficiency, and wettability. This technology is widely applied in pesticide spraying, inkjet printing, and electrostatic oil coating.

Electrostatic atomization for minimum quantity lubrication(EMQL): Building on pneumatic MQL, EMQL uses high-voltage electrostatics to atomize the lubricant into charged micro-droplets, transported to the cutting area for enhanced cooling and lubrication [217]. Studies show that EMQL outperforms pneumatic MQL in atomization quality and machining efficiency [149]. Reddy et al. [218] pioneered electrostatic atomization in drilling, achieving improved cutting forces and surface quality.

The EMQL system is shown schematically in Figure 20a. Compressed air and cutting fluid pass through an insulated nozzle and are charged by a high-voltage generator. The charged fluid atomizes into an oil mist via high-pressure airflow, which is sprayed onto the grinding wheel and workpiece. Figure 20b illustrates the electrostatic atomization process: charged droplets become surface-unstable, increasing susceptibility to secondary atomization [149]. Ideally, droplets maintain a spherical shape under surface tension P, with internal and external static pressures Pe (Figure 20c). When charged, electrons concentrate on the droplet surface due to the skin effect, creating a repulsive force Pe that counteracts surface tension, leading to electrostatic expansion and reduced surface tension [219, 220]. This decreases the droplet contact angle [221](Figure 20d). When electrostatic repulsion equals the surface tension, droplets break apart [222]. EMQL produces finer droplets that penetrate the cutting zone, forming a denser, more stable lubrication film.

Figure 20
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(a) Components of the electrostatic atomization system, (b) The principle of electrostatic atomization, Effect of electric field on surface tension (c) and wettability (d) of droplets

Bio-lubricants high viscosity and surface tension limit heat transfer and diffusion to the cutting zone. However, charged droplets improve these limitations. Charging has minimal effect on droplet viscosity, thus preserving lubrication performance. Nanoparticles further enhance bio-lubricants' atomization and charging characteristics, optimizing EMQL for machining.

Electrostatic atomization can produce smaller, more uniformly distributed droplets under reduced air pressure, preventing droplets from dispersing outside the deposition area due to pressure fluctuations. The charge on these droplets increases surface activity and lowers surface tension [149], significantly enhancing their diffusion within the cutting zone (Figure 20d). Researchers have explored the effects of parameters such as atomization cone angle, droplet size, base fluid properties, and electrode-to-nozzle distance on the atomization process.

The sprayed mist forms an atomization cone at the nozzle outlet, where the tangent of the atomization cone angle (Figure 20a) indicates the ratio of the radial average velocity of the droplets to the axial average velocity. This parameter is crucial for atomization performance. Zhang et al. [223] studied the relationship between the atomization cone angle and voltage amplitude (Figure 21a). When the voltage is below 50 kV, increasing the voltage raises the electric field strength near the needle electrode, leading to greater fragmentation of the droplets and an increase in their charge. Consequently, the Coulomb force acting between the charged droplets enhances their radial velocity. When a voltage amplitude of 60 kV is applied, the atomization cone angle decreases, indicating that at higher voltages, the increased electric field strength between the needle and plate causes the radial electric field force on the charged droplets to dominate. This results in the axial velocity exceeding the radial velocity, reducing the time the charged droplets are in motion and causing them to quickly move from the nozzle outlet to the workpiece surface, which decreases the atomization cone angle.

Figure 21
Figure 21
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(a) The relationship between the atomization cone angle and the voltage amplitude [223], (b) The relationship between the distance between electrodes and droplet size [223], (c) The relationship between voltage amplitude and cutting material removal (CMR) at different mass fractions [219], (d) Spray particle size distribution span under different working conditions

The relationship between droplet size and voltage is shown in Figure 21d, where droplet size decreases with increasing voltage. However, beyond a certain voltage range, as the voltage continues to rise, the speed of the droplets also increases. Within a limited space and time, the number of times droplets can split is finite, leading to a leveling off of changes in droplet size. This indicates that the relationship between droplet size and voltage is nonlinear. Lv et al. [220] experimentally confirmed this relationship: within a certain voltage range, droplet size decreases as voltage increases. Jia et al. [224] noted that during machining, intermittent discharge breakdowns can occur between the nozzle and the grinding wheel when the voltage reaches 40 kV, posing significant safety hazards.

The distance between electrodes affects droplet size, as shown in Figure 21b. Droplet size decreases with greater electrode spacing, as increased distance supports better atomization. However, at distances over 120 mm, droplet size stabilizes due to reduced electric field strength near the electrode needle, which diminishes the corona effect, thus leveling droplet size [223].

The atomization effectiveness in electrostatic systems largely depends on the charge capacity of the droplets, characterized by the Charge-to-Mass Ratio (CMR, CMR=Q/m) [225]. Higher CMR values improve the charge-carrying ability of droplets, accelerating droplet breakup and improving atomization performance. Critical CMR values are inversely related to particle size, so adding nanoparticles to lubricants enhances CMR. Figure 21c shows that as nanoparticle volume fraction increases, nano-bio lubricants exhibit higher charge capacity and CMR [219].

Research on nozzle atomization quality also focuses on droplet size distribution. A concentrated droplet distribution allows better control over atomization parameters. The droplet size distribution span, represented by R.S., increases with a wider size range, resulting in less uniform droplet production. Jia et al. found that higher voltage improves droplet size uniformity by causing the liquid film at the nozzle to spread more evenly, intensifying lateral fluctuations and creating denser lateral wave crests. This leads to a more uniform droplet size distribution, reducing the likelihood of extreme droplet sizes. Within the effective spraying range of MQL, charging the liquid notably enhances droplet size uniformity (Figure 21d).

Jia et al. [226] enhanced electrostatic atomization efficiency by adding lecithin to biolubricants. Lecithin increased soybean oil's electrical conductivity, viscosity, and charge-to-mass ratio while reducing surface tension. Comparative experiments showed that MQL led to a maximum workpiece surface temperature rise of 241.6°C. Under flooding, EMQL, and L+EMQL conditions, surface temperatures dropped by 28.97%, 42.8%, and 48.92%, respectively, compared to MQL (Figure 22a). The lowest grinding force was observed with L+EMQL (Figure 22b). Yang et al. [228] proposed a conical jet droplet size distribution model and confirmed experimentally that electrostatic atomization significantly improves droplet size control compared to pneumatic methods. He et al. [227] reported that during Ti-6Al-4V grinding, dry grinding resulted in severe surface adhesion and gouging. In contrast, EMQL conditions produced a workpiece surface free of adhesive re-cast layers, similar to the optimal surface achieved with MQL (Figure 22c). Lin et al. [229] analyzed the grinding surface of 45 steel and observed that EMQL at 4 kV voltage produced a smoother finish than MQL. Jia et al. [230] found that EMQL achieved the best surface quality for high-temperature nickel-based alloy GH4169, reducing Ra and RSm values by 31.1% and 37.3% compared to dry grinding and by 17.1% and 25% compared to MQL. In Cr12 grinding, EMQL showed a higher G-value than MQL within a specific voltage range.

Figure 22
Figure 22
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Heat and specific grinding force under different lubrication conditions: (a) Heat, (b) Specific grinding force, (c) SEM of images of the machined surface [226, 227]

Xu et al. [231] found that the crystal structure of a workpiece changed with voltage during EMQL processing (Figure 23b). In the images, dark crystals indicate sorbite, while white ones represent alloy cementite. At 5 kV, cementite content on the workpiece surface peaked, as negatively charged vacancies in the metal material were attracted to the positive electrostatic field of EMQL, migrating to the surface along grain boundaries. This promoted dislocation deposition on the surface, lowering microhardness. Simultaneously, electric and thermal effects caused iron (Fe) atoms in cementite to be replaced by chromium (Cr) and carbon (C), forming alloy cementite that increased surface concentrations of Cr and C (Figure 23a).

Figure 23
Figure 23
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(a) The influence of  voltage static electricity on the processed surface (b) Processing surface under different voltages [231]

4.3.3 Ultrasonic Vibration-Enhanced MQL

Ultrasonic-assisted machining combines ultrasonic vibration with traditional machining to alter material removal mechanisms, improve processes, and enhance results [232,233,234]. The impact of ultrasonic vibration varies with direction [235,236,237]. Ultrasound affects the tool path, resulting in improved machined surface quality (Figure 24).

Figure 24
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Ultrasonic assisted grinding principle

Chen et al. [238] used ultrasonic-assisted dry grinding on 12Cr2Ni4A, achieving a 30% reduction in surface roughness compared to conventional dry grinding. Bie et al. [239] studied ultrasonic vibration-assisted dry gear grinding, observing 40% and 30% reductions in tangential and normal grinding forces, respectively, and a 25.6% decrease in surface roughness compared to conventional gear grinding (CGG). However, ultrasonic-assisted dry grinding alone is inadequate for lubricating challenging workpieces, and it fails to meet green processing standards. Consequently, ultrasonic vibration-assisted grinding based on MQL technology has been proposed [240,241,242,243,244].

Ultrasonic vibration can break droplets into smaller sizes, known as ultrasonic atomization [245, 246], used in spray cooling, aircraft de-icing, and pesticide dispersion [247, 248]. Ultrasonic atomization affects droplets through capillary waves [249,250,251] and cavitation [252,253,254]. Upon contact with a vibrating surface, capillary waves form at the liquid edge, spreading toward the center (black arrows in the first row and yellow arrows in the second row of Figure 25d). As impact velocity or vibration amplitude rises, bubbles form inside (Figure 25d), creating hydraulic shocks that accelerate droplet dispersion and splashing [255,256,257,258].

Figure 25
Figure 25
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Droplet reflection under the influence of ultrasound [263]

Vertical vibrations induce capillary waves [259, 260]. As ultrasonic vibration amplitude increases, the liquid column breaks, ejecting secondary droplets from wave peaks (Figure 25a), a phenomenon called "Faraday instability" [261, 262]. Droplet splashing is accelerated by gas overpressure as droplets and gas overflow from the vibrating surface (Figure 25b). High amplitudes and strong cavitation increase capillary wave intensity, leading to more uneven waves and intense droplet splashing (Figure 25c).

Zhang et al. [263] observed that ultrasonic vibration promotes droplet atomization by inducing capillary waves that converge toward the droplet center, causing rebound and splashing (Figure 25d).

Das et al. [240] compared dry and MQL grinding with ultrasonic assistance. As shown in Figure 26, dry cutting exhibited the highest tangential and normal forces, while ultrasonic-assisted grinding reduced both grinding force and surface roughness. MQL forms a lubricating film in the grinding area, reducing friction. Compared to dry grinding, UGA+MQL decreased tangential and normal forces by 42% and 31%, respectively. Ultrasonic-assisted technology is particularly beneficial for brittle materials, as it reduces fracture and achieves optimal surface quality through ductile removal.

Figure 26
Figure 26
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Tangential and normal forces under four different grinding conditions [240]

Gao et al. [264] investigated the effects of ultrasonic angle on grinding using a two-dimensional ultrasonic vibration-assisted device (Figure 27a). Tangential ultrasonic vibration-assisted grinding primarily improves surface quality through repeated polishing actions, while axial ultrasonic vibration widens grooves and reduces the height and width of surface protrusions [265]. When the angle θ between the axial and tangential ultrasonic transducers changes from 45° to 135°, the suction effect shows a nonlinear trend: it initially decreases, then increases as θ increases. A stronger suction effect enhances the axial guiding ability of the lubricating fluid. As shown in Figure 27b, when θ changes from 45° to 90°, surface roughness Ra during grinding with Al2O3/SiC mixed nanofluid under two-dimensional ultrasonic vibration increases, mirroring the trend observed in pouring lubrication. The lowest Ra is achieved at 45°. Additionally, Al2O3 and SiC nanoparticles, being hard, create a polishing effect when cavitation bubbles collapse, further enhancing surface quality.

Figure 27
Figure 27
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a Schematic diagram of two-dimensional ultrasonic vibration assistance, b The effect of angle on Ra under different conditions [264]

Rabiei et al. [244] added Al2O3 nanoparticles to the MQL base fluid combined with ultrasonic vibration during grinding, resulting in a 20% reduction in both specific and tangential grinding forces, a 48% decrease in temperature, and no surface burn. Molaie et al. [266] found that MoS2-based nano-biological lubricants reduced tangential force by 60% under horizontal ultrasound. Sun et al. [267] applied ultrasonic-assisted grinding to bone, observing that ultrasound reduced maximum undeformed chip thickness, lowered material hardness and fracture toughness, and thereby facilitated bone removal whereas reducing grinding force. Yang et al. [51] employed ultrasonic-assisted nano-biological lubricant MQL for bone grinding. Compared to dry grinding, ultrasonic vibration, MQL, and NMQL alone, friction coefficients decreased by 31.3%, 19.0%, 9.8%, and 12.5%, respectively, whereas grinding temperature dropped by 33.5% and 10.0% compared to using ultrasonic vibration or NMQL alone. Dambatta et al. [268] observed that adding SiO2 nanofluids to the MQL system reduced specific grinding energy by up to 65% during ultrasonic vibratory milling.

4.3.4 Magnetic Traction Nano-lubrication

Magnetorheological (MR) processes, commonly used in surface polishing, utilize magnetic fields to control the movement of magnetic fluids [269,270,271,272]. To improve the wettability of nano-biological lubricants in the grinding area and optimize nanoparticle alignment, Cui et al. [273] developed a method for magnetically driven nano-fluid lubrication based on NMQL technology.

The principle is illustrated in Figure 28a. Permanent magnets placed on either side of the grinding wheel create a magnetic field gradient across its surface (Figure 28b) [273]. This magnetic force forms a lubricating film from magnetic nano-micro droplets, facilitating nanoparticle entry into the grinding area and enhancing micro-droplet adsorption on the wheel surface, thus improving lubricant cooling and lubrication performance.

Figure 28
Figure 28
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(a) Magnetic field-assisted device, (b) Magnetic field distribution simulation [273]

The study used Fe3O4 and graphene (2:1) mixed nanoparticles for grinding Ti-6Al-4V, as graphene provides good lubrication properties, while Fe3O4 imparts magnetic characteristics to the fluid. As shown in Figure 29a, adding graphene (GR) alone did not significantly affect grinding force and temperature in magnetic or non-magnetic conditions. However, the addition of Fe3O4 nanoparticles significantly reduced grinding force and temperature under magnetic influence, demonstrating the magnetic field's effectiveness in assisted grinding. Compared to non-magnetic conditions, normal and tangential grinding forces were reduced by 17% and 31.5%, respectively, with Fe3O4/GR mixed nanoparticles. Figure 29b shows grinding temperature curves from three measurement channels along the grinding length. Channels 1, 2, and 3 correspond to the lowest to highest positions. Peak temperatures at the grinding zone (Channel 3) and the temperature difference between Channels 3 and 1 reflect lubricant penetration, with the magnetic field enhancing this effect [274, 275].

Figure 29
Figure 29
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(a) Grinding force under different lubrication conditions, (b) Grinding temperature under different lubrication conditions [273]

4.4 Other Clear Grinding

In addition to MQL and its related technologies, other clean grinding methods include cryogenic cooling (discussed in Section 4.3.1), dry grinding, and solid lubrication grinding. Among these, cryogenic grinding constitutes 36% of publications, slightly less than solid lubrication (Figure 30). Dry grinding has the smallest share at 27%, likely due to its comparatively limited performance.

Figure 30
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Proportion of other grinding types

4.4.1 Dry Grinding

Dry machining has gained global research interest as it eliminates cutting fluids, reducing environmental hazards and operational costs [86]. By removing the need for cooling liquid systems and their components, such as high-pressure pumps, filtration systems, cooling tanks, and magnetic nozzles, dry grinding eliminates the expenses associated with metalworking fluid treatment facilities and bacterial control [276, 277].

One approach to improving dry grinding performance involves specially dressed wheels. In conventional grinding, many abrasive grains contact the workpiece surface, yet only a fraction actively contribute to material removal, while others create friction that increases heat without cutting [61,62,63, 87]. Special dressing techniques can reduce contact between the wheel and the workpiece.

Tawakoli et al. [278] used a single-point CVD diamond dressing tool to perform structured dressing on CBN resin-bonded wheels for dry grinding 100Cr6 steel. They adjusted the wheel structure to reduce the CBN contact layer by 25%, 50%, and 75%, such that, for example, a 25% structured wheel means that 25% of the surface no longer contacts the workpiece (Figure 31a). With conventional wheels, burning and thermal damage of at least 0.01 mm depth were observed, whereas the 25% structured wheel eliminated burning (Figure 31b). Azarhoushang et al. [279] conducted plunge external dry grinding using wheels with structuring levels of 30%, 60%, and 75%, finding that structured wheels improved all process characteristics, except wheel wear, compared to fully abrasive wheels. However, structured wheels showed slightly worse surface roughness than conventional wheels.

Figure 31
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(a) Dressed wheel, (b) Surface machined with dressed wheel [278]

He et al. [280] proposed a ring-shaped rotating heat pipe structure within the grinding wheel substrate to enhance its thermal conductivity, allowing efficient heat dissipation from the grinding arc region. Figure 32a illustrates the heat pipe wheel structure, and Figure 32b shows the heat exchange mechanism in an axial rotating heat pipe wheel. To minimize thermal resistance, the heat pipe is embedded directly within the wheel substrate. The abrasive-covered grinding surface functions as the evaporation section, while the upper finned area serves as the condensation section, with an insulation section between them.

Figure 32
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(a) Heat pipe grinding wheel structure diagram, (b) Schematic diagram of heat exchange of heat pipe grinding wheel [280]

During grinding, centrifugal force distributes the working fluid along the inner walls of the heat pipe's evaporation and insulation sections [280, 281]. Grinding heat transfers through abrasives and the substrate to the heat pipe's evaporation section. Here, the fluid absorbs heat, undergoes a phase change, and vaporizes, moving toward the condensation section due to pressure differences. Low-temperature air jets cool the condensation section, releasing heat from the fluid, which then condenses. The conical shape of the condensation section aids the condensed fluid's return to the evaporation section via gravity and centrifugal force, forming a continuous thermal cycle that enhances heat transfer in the grinding process [282, 283].

In creep feed grinding experiments on Inconel 718, the heat pipe wheel maintained grinding temperatures consistently below 100°C without any additional coolant, producing better surface quality than conventional wheels [284]. In dry grinding of carbon steel, the heat pipe wheel reduced grinding temperatures by 50% under specific conditions [280]. The heat pipe wheel technology has the potential to reduce or eliminate harmful coolants, creating an eco-friendly grinding solution. Compared to conventional wheels, heat pipe wheels decrease energy consumption by 26%–42% and reduce carbon emissions by 45%–56% [285].

4.4.2 Solid Lubrication

Solid lubrication involves using solid materials to reduce friction and wear on contact surfaces [286,287,288,289,290,291,292]. Common solid lubricants include graphite, molybdenum disulfide (MoS2), tungsten disulfide, and calcium sulfate [293,294,295]. These materials are particularly suited for grinding due to their layered structures, low shear strength, and ability to adsorb onto workpieces [64, 296,297,298]. MoS2 and graphite are especially popular for their unique interlayer structures and crystal properties, offering excellent lubrication [299,300,301].

Solid lubrication is generally applied in two ways. The first method uses feeders to introduce powder-form solid lubricants directly to the grinding area [302]. However, this requires additional cleaning of the workpiece and tool and involves extra equipment, which can limit its practicality. The second method integrates the solid lubricant within the grinding wheel itself, enabling self-lubrication during grinding [303, 304].

Alberts et al. [88] used graphite as a solid lubricant in grinding D-2 tool steel, observing a significant reduction in grinding force and grinding ratio. Mahathanabodee et al. [306] incorporated hexagonal boron nitride as a solid lubricant into 316L stainless steel composites, which enhanced the material's frictional properties. Zhao et al. [305] prepared MoS2-embedded metal composites via liquid phase sintering, finding that MoS2 samples provided improved mechanical and tribological properties at similar concentrations.

In another study, Zhao et al. [305] coated cubic boron nitride (CBN) grinding wheels with MoS2 for dry grinding titanium alloys (Figure 33a). The MoS2 coating formed a lubricating film on the wheel surface, reducing normal grinding force by 42.5% and tangential force by 28.1%. Under specific conditions, the grinding temperature was significantly lower than that achieved with conventional wheels, which reduced chip adhesion and extended the wheel's lifespan (Figure 33b).

Figure 33
Figure 33
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(a) Coated grinding wheel, (b) Grinding temperature versus grinding speed [305]

Ding et al. [307] developed a self-lubricating grinding wheel (Figure 34), in which solid powder lubricants are ejected from an internal cavity onto the grinding surface via centrifugal force. This design enhances the wheel's friction and wear resistance during grinding. After grinding TC4 alloy, the abrasive grains remained uniformly sized, with no spalling or chipping.

Figure 34
Figure 34
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Internally sprayed self-lubricating grinding discs (a) and wheels (b) [307]

5 Conclusions

This paper presents a comprehensive analysis of environmentally friendly grinding processes, highlighting emerging trends and potential directions for future research. Utilizing VoSviewer, R Studio, and Microsoft Excel, we analyzed 738 publications from the WOS database spanning 2008 to 2023. The study reviews green grinding technologies, examining both their mechanisms and current applications. These findings are intended to assist researchers in this field, with the main conclusions summarized as follows:

  1. (1)

    Publication Trends: The volume of publications related to environmentally friendly grinding has increased annually, with an average growth rate of 8.34%. Among the 118 countries publishing on this topic, China leads with 45.38% of publications, followed by India with 17.44%. Leading institutions in this field are based in China, India, and Brazil. EC Bianchi from Brazil has the highest number of publications (81), whereas CH Li from China, with 80 publications, holds the highest H/G indices at 48 and 80, respectively. Additionally, 70% of the top ten journals by publication volume are ranked as Q2 or higher.

  2. (2)

    Keyword Analysis: "MQL" appears 337 times as a keyword, and the most frequently cited papers are related to MQL, underscoring its prominence as a favored environmentally friendly grinding technology from 2008 to 2023. Other hot keywords, such as "plant oils" and "nanofluids," reflect growing research interest in these technologies.

  3. (3)

    Biolubricants: Biolubricants offer excellent cooling and lubrication due to their fatty acid content, which can form a lubricating film through saponification reactions with metal surfaces. Fatty acids can be classified into saturated and unsaturated types. Although unsaturated fatty acids provide better viscosity and surface tension, their C=C double bonds are prone to oxidation, leading to performance degradation. Longer-chain fatty acids produce stronger lubricating films due to higher intermolecular cohesion; for instance, rapeseed oil (C22) outperforms soybean oil (C18). For saturated fatty acids, the lubrication effect plateaus when the carbon chain reaches 16 atoms. Blending biolubricants with different viscosities can enhance performance; for example, a 1:1 mixture of soybean and castor oils reduces the tangential grinding force by 27% when grinding GH4169 compared to castor oil alone.

  4. (4)

    Nanoparticles in Biolubricants: Adding nanoparticles to plant-based lubricants can improve lubrication and heat dissipation. Different nanoparticle shapes affect lubrication differently. Spherical nanoparticles tend to form bearing-like structures, while onion-like nanoparticles shed layers as they roll, and layered nanoparticles enhance lubrication through layer shearing. A MoS2 and CNT mixture (2:1) improves lubricant performance by addressing MoS2's tendency to fail and CNT aggregation, thereby enhancing stability and effectiveness.

  5. (5)

    Advanced MQL-Based Techniques: To address extreme boundary conditions, various MQL-based methods have been developed. In CMQL, the surface tension of plant oil droplets rises at low temperatures, reducing wettability in the grinding zone but increasing lubricant viscosity and oil film thickness, which enhances load capacity, prevents tool failure, and reduces surface burning. Electrostatic enhancement disperses droplets more effectively, lowering surface tension and contact angle to improve wettability. Ultrasonic-assisted MQL disperses mist droplets, driving nanoparticles to impact the processing surface and improving surface quality. Magnetic field enhancement directs nanoparticles into the grinding zone; for example, using a Fe3O4 and GR mixture (2:1) on titanium alloys can reduce grinding force by up to 30%, thereby improving efficiency and surface quality.

  6. (6)

    Dry Grinding and Solid Lubrication: Advances in wheel surface morphology and coatings with solid lubricants reduce grinding force in dry grinding and solid lubrication grinding. When grinding carbon steel, thermal tube wheels can lower the grinding temperature by up to 50%. However, their effectiveness for machining challenging materials has not yet been demonstrated in published research.

6 Prospects

Research into green grinding technologies has been ongoing for decades. Since the last century, methods such as ultrasonic-assisted dry grinding, solid lubrication grinding, liquid nitrogen cooling, low-temperature airflow impact cooling, and MQL grinding have been proposed. Due to factors such as equipment, cost, and precision, MQL grinding has emerged as the most widely applied technique in the industry. Based on current advancements, MQL and its related techniques are expected to lead future green grinding innovations. Potential research directions include:

  1. (1)

    Optimizing MQL Base Fluids: Vegetable oils, commonly used as MQL base fluids, are prone to decomposition at high temperatures due to C=C double bonds, resulting in unstable lubrication performance. Adding nanoparticles to these fluids can improve grinding performance, as different nanoparticles offer different advantages. Establishing a database to guide optimal nanoparticle selection, considering factors like CGN's high friction reduction but limited penetration, will be essential for advancing MQL technology.

  2. (2)

    Multi-Energy Field Enhancement: Different machining methods and materials require specific processing parameters, especially for difficult-to-machine materials. Matching these parameters with energy field enhancements is an area worth exploring. How to balance the enhanced cooling effect in cryogenic grinding with the potential for complex stress-induced reactions in the workpiece material. It is necessary to quantitatively evaluate the impact of ultra-low temperature cooling on the material’s performance and determine the threshold at which ultra-low temperature media affect different materials. In electrostatic enhancement, high-voltage electrostatics can alter surface crystals, yet the parameters influencing these changes remain unclear. Magnetic field enhancement is also a relatively new field with limited research. Future studies should aim to identify optimal boundary conditions for magnetic field enhancements, which could improve the efficiency and quality of machining challenging materials.