Elsevier

Ceramics International

Volume 46, Issue 3, 15 February 2020, Pages 3582-3591
Ceramics International

Investigating Minimum Quantity Lubrication in Unidirectional Cf/SiC composite grinding

https://doi.org/10.1016/j.ceramint.2019.10.076Get rights and content

Abstract

The influences of minimum quantity lubrication (MQL) on the grinding performance of unidirectional carbon fibre-reinforced ceramic matrix (Cf/SiC) composites were investigated in this paper. The experimental results indicate that MQL can significantly improve surface quality and reduce grinding forces. In addition, MQL has a low cost and does not generate considerable pollution. According to the lubrication mechanism of MQL, the effects of nozzle direction, air pressure, oil flow rate and nozzle distance on the grinding performance of unidirectional Cf/SiC composites were carefully investigated in this research. Excellent surface quality and low grinding forces can be achieved when the nozzle direction, air pressure, oil flow rate and nozzle distance are 15°, 5 bar, 100 ml/h and 80 mm, respectively. The surface topographies show that smooth fibre separation, fibre breaking, fibre outcropping, fibre pullout and matrix cracks are the main failure forms of the ground surface. The histogram reflects that the proportion of fibre pullout is the highest, whereas that of fibre outcropping is lowest. In the MQL grinding process, a large amount of heat is removed by water vapour, which can significantly reduce the grinding temperature. Meanwhile, effective oil films form in the contact areas between the grits and the material surface. The above factors are advantageous for improving the grinding performance of unidirectional Cf/SiC composites. The objective of this research was to investigate the influences of MQL for unidirectional Cf/SiC composites, validate these interpretations with experimental results and provide reasonable recommendations for guaranteeing the machining accuracy of Cf/SiC composites.

Introduction

Ceramic matrix composites (CMCs) exhibit excellent corrosion and chemical resistance properties and are extensively utilized for a vast range of applications in space propulsion systems and braking systems [[1], [2], [3]]. As typical CMCs, carbon fibre-reinforced ceramic matrix (Cf/SiC) composites are the most promising class of materials, with the potential to be extensively applied in industry because of their outstanding performance in complex and harsh environments [4].

A common preparation method for Cf/SiC composites is chemical vapour deposition under high temperatures [5]. Large quantities of silicon carbide are deposited on the carbon fibres in this preparation process. However, the surface quality of Cf/SiC composites cannot meet industry requirements. Currently, grinding with a diamond wheel is the most efficient surface processing method [6]. Qu et al. [7] researched the influence of fibre direction on the grinding characteristics of unidirectional Cf/SiC composites and concluded that the fibre direction determines the material removal mechanisms. Single-grain scratching trials were carried out on three typical surfaces of two-dimensional Cf/C–SiC composites by Liu et al. [8]. The results show that the scratching forces follow the order of normal > transverse > longitudinal, and brittle failure is the basic removal form. To more accurately reflect the ground surface quality, Cao et al. [9] proposed three-dimensional surface evaluation parameters based on anisotropic and non-homogeneous structural characteristics.

However, because of the considerable hardness and low heat conductance of Cf/SiC composites, a considerable amount of heat would be absorbed and stored by the grinding wheel in the dry grinding process, indicating that the number of surface grains, the sharpness and the service life of the wheel would be reduced. In addition, due to the passivation of a grinding wheel, the surface quality of the machined surface decreases [10]. Thus, effective cooling technology should be applied in the grinding process. Flood delivery with a low velocity and high volume is a conventional cooling method [11]. Flood grinding can reduce the friction and heat in the grinding area. For Cf/SiC composites, the grinding chips are small and drift into the air during the dry grinding process. To ensure safe working conditions for workers, grinding fluid should be utilized when grinding Cf/SiC composites to wash away chips from the grinding area [12]. Furthermore, conventional grinding fluids are characterized by significantly high costs and low efficiencies. Due to their molecular structural characteristics, grinding fluids are degraded in the environment with difficultly. When grinding fluids are unreasonably placed, serious ecological problems, such as water and soil pollution, can result [13]. More seriously, toxic fumes can liberate from grinding fluids, leading to serious diseases in those who have been exposed [14].

To ensure processing quality and reduce occupational hazards, minimum quantity lubrication (MQL) grinding has been utilized in the processing of difficult-to-machine materials. In the MQL processing system, an aerosol is delivered into the processing area. In the same amount of time, the consumption of conventional grinding fluid is nearly 1000 times that of MQL fluid (in volume) [11]. Based on the previous literature [[10], [11], [12], [13]], MQL is an efficient, safe and feasible cooling and lubrication method. The grinding characteristics of AISI 52100 steel under different cooling parameters were studied by Mao et al. [15]. The results show that MQL can significantly improve the ground surface quality. Based on the working principle of MQL, Tawakoli et al. [16,17] researched the effects of aerosol conditions on surface finish and grinding force. The results show that the grinding quality can be improved when the angle between the aerosol injection direction and workpiece surface is approximately 10–20°. Regarding the grinding process of Al2O3, theoretical research of the MQL delivery parameters was conducted by Emami et al. [18,19], who indicate that when the nozzle angle and distance are 15° and 30 mm, respectively, the cooling and lubrication effects are the best. The influence of dry cooling, MQL and nanoparticle MQL on the grinding forces and surface quality of ZrO2 was studied by Yang et al. [20]. The results show that the maximum undeformed chip thickness of the ductile-brittle transition would be increased in the MQL and nanoparticle MQL conditions. To further reduce the consumption of MQL fluid, higher pour point lubricants were utilized by Garcia et al. [21]. This research found that a CO2 nozzle can supply the minimum amount of gas to freeze aerosols. The basic principle is that only a thin fluid film provides effective action. Although the MQL strategy significantly reduces grinding fluid consumption, the grinding wheel is easily clogged. A method of combining the MQL system and a cleaning nozzle was proposed by Lopes et al. [22]. When the inclination angle of the air jet is 30° from the ground surface, the wear of the wheel is delayed, and the grinding chips on the surface of the grinding wheel can be effectively cleared. To meet processing requirements and reduce costs, grinding Cf/SiC composites with MQL were proposed by Adibi et al. [3,23]. The influences of grinding speed, feed speed and grinding depth on the grinding performance of Cf/SiC composites under the unified MQL conditions have been investigated. However, the influences of the MQL parameters have not been researched. According to the experimental results and theoretical equations, the optimal combination of grinding and lubrication conditions is proposed and validated. Meanwhile, scanning electron microscopy (SEM) images of the ground surface reflect that the fundamental mechanism of Cf/SiC composite grinding is brittle fracture. Until now, many MQL studies have researched the influence of MQL on the machining performance of conventional materials. For Cf/SiC composites, although many studies have been carried out under dry or wet conditions, very few experiments have been performed to investigate the influence of MQL grinding on the grinding performance of Cf/SiC composites.

Cf/SiC composites, a new class of structural materials, have been researched by many scholars. However, how to achieve a good surface quality and analyse the grinding mechanism are the primary concerns discussed in most studies. Based on previous studies, a large amount of fine grinding chips can drift into the air during the dry grinding process, which has major and pernicious health implications for workers. Although grinding chips can be effectively washed away by grinding fluid, the purchasing and disposal costs of grinding fluid greatly improve the grinding cost. In this paper, MQL is utilized in Cf/SiC composite grinding as an alternative cooling and lubrication method. More importantly, only a few studies have investigated the influence of MQL on the grinding process of Cf/SiC composites. This research aims to meticulously investigate the influences of the MQL conditions on the surface quality and grinding forces. In addition, the grinding mechanisms of the Cf/SiC composites are researched based on the ground surface topography. The research conclusions are useful for optimizing MQL parameters and improving the grinding efficiency and quality of grinding Cf/SiC composites.

Section snippets

Material and sample preparation

According to the direction of greatest required strength, fibre bundles must be laid or woven prior to sample preparation. Depending on the fibre types and structures, ceramic matrix composites (CMCs) can be divided into many types, and each type has different functions and prices. As the primary objective of this paper is to investigate the effects of MQL and grinding conditions on the grinding machining properties of Cf/SiC composites, unidirectional Cf/SiC composites with easy decoupling are

Effects of lubrication conditions on grinding performance

To investigate the effects of the lubrication conditions on the grinding performance, dry grinding, wet grinding and MQL grinding were compared. The grinding forces and surface roughnesses under different lubrication conditions are shown in Fig. 3, Fig. 4. The values of surface roughness and grinding force under wet and MQL conditions were significantly lower than those with dry grinding, indicating that wet and MQL grinding can improve the grinding performance of Cf/SiC composites over that

Grinding mechanism

The grinding mechanism of Cf/SiC composites, typical multiphase materials, is significantly different than the grinding mechanism of conventional metal or ceramic materials. The failure processes of the matrix and carbon fibre reinforcement phases occur synchronously due to the structural characteristics of Cf/SiC composites, which lead to a more complex grinding mechanism. The analysis of the grinding mechanism helps to identify and gain a better understanding of the grinding defects.

Conclusions

The effects of the MQL parameters on the surface roughness and grinding force of unidirectional Cf/SiC composites were investigated in this paper. According to the working principles of MQL, nozzle direction, air pressure, oil flow rate and nozzle distance were the primary analysis parameters considered. In addition, the material removal mechanism of unidirectional Cf/SiC composites was carefully investigated via micrograph images of the ground surface. The function of oil droplets was also

Recommendations for future work

In this paper, the influences of MQL were evaluated by grinding Cf/SiC composites. However, the MQL working mechanism and effects on Cf/SiC composites, a novel functional material, have not been thoroughly researched thus far. According to the results of this paper, more research should be carried out to better investigate the MQL working mechanism in grinding Cf/SiC composites. For example, the effects of lubricating oil type on the grinding performance of Cf/SiC composites should be carefully

Declaration of competing interest

We would like to submit the enclosed manuscript entitled “Investigating Minimum Quantity Lubrication in Unidirectional Cf/SiC Composite Grinding”, which we wish to be considered for publication in “Ceramics International”. No conflict of interest exits in the submission of this manuscript, and manuscript is approved by all authors for publication. I would like to declare on behalf of my co-authors that the work described was original research that has not been published previously, and not

Acknowledgements

The author wishes to thank the National Natural Science Foundation of China (No. 51775100) and Fundamental Research Funds for the Central Universities in China (No. N180306001 and N180303028) for financial assistance.

References (29)

Cited by (0)

View full text