Guangchun Xiao

Investigation of Al2O3/TiC ceramic cutting tool materials with the addition of SiC-coated h-BN: preparation, mechanical properties, microstructure and wear resistance

Abstract SiC-coated h-BN powders ((h-BN)/SiC) were used to substitute h-BN as solid lubricant to prepare Al2O3/TiC/(h-BN)/SiC self-lubricating ceramic cutting tool materials using hot pressing. Microstructure and mechanical properties of the material were studied. Overall improvements in Vickers hardness and fracture toughness were obtained compared with that of Al2O3/TiC/h-BN. The coating of SiC blocks direct contact between h-BN and Al2O3/TiC and hence enables h-BN particles to bond more closely with Al2O3/TiC. The density of the material was dramatically improved because of the decrement of voids around h-BN particles. Dry turning tests were carried out to investigate the wear resistance of the materials when cutting 40Cr quenched and tempered steel. Enhancement in wear resistance, which may result from the improvement in mechanical properties, was observed.

Recent Progress in Self-Lubricating Ceramic Composites

Structural ceramic composites have received increasing attention over the past few decades for their potential applications in various fields. Lubrication is usually required for moving ceramic parts because of their high coefficient of friction under dry sliding conditions. Self-lubricating ceramic composites have been applied in severe operating conditions where conventional lubrication method, such as liquid lubrication, is unavailable. The solid lubricants added in self-lubricating ceramic composites can reduce the coefficient of friction. However, they decrease mechanical properties and then weaken antiwear property of the ceramic composites, which consequently restricts self-lubricating ceramic composites’ application scope. Therefore, there is a contradiction between the antifriction and antiwear properties of self-lubricating ceramic composites and many efforts from researchers have been devoted to resolve it. In this chapter, two new types of self-lubricating ceramic composites were elaborated. Graded self-lubricating ceramic composites were developed by adopting the design concept of functionally graded materials (FGMs). Their characteristics are that the solid lubricant content decreases with a gradient from the surface to the center and thermal residual compressive stresses exist in the surface after the sintering process. The gradient distribution of solid lubricant and the thermal residual compressive stresses are used to improve the mechanical properties of the ceramic composites. Another new type of self-lubricating ceramic composites is those with the addition of coated solid lubricants. The solid lubricant powders are firstly coated by metal or metallic oxide, etc., to form core-shell structured composite powders and then mixed with the ceramic matrix powders to prepare self-lubricating ceramic composites by sintering. The shell substance is used to protect the solid lubricant core from reacting with the ceramic matrix during the sintering process and promote the relative density of the ceramic composites. The two new types of self-lubricating ceramic composites showed superior mechanical properties and tribological properties to the traditional self-lubricating ceramic composites.

Lubrication Performance of Graphene as Lubricant Additive in 4-n-pentyl-4′-cyanobiphyl Liquid Crystal (5CB) for Steel/Steel Contacts

The lubrication performance of graphene used as additive in 4-n-pentyl-4′-cyanobiphyl liquid crystal (5CB) for steel/steel contacts was studied on a ball-on-plate tribotester. The friction test results show that when the graphene content in the 5CB was 0.15 wt.%, and the lubricant and friction pairs were heated to 44–46 °C before friction tests, the lubrication performance of the 5CB was most improved. Compared with pure 5CB, 5CB+0.15 wt.% graphene suspension reduced the friction coefficient and wear scar diameter by up to 70.6% and 41.3%, respectively. The lubrication mechanisms have been tentatively proposed according to the test results. We speculate that the excellent lubrication performance of graphene/5CB suspensions may be attributed to the low shear resistance adsorption layer formed by graphene and 5CB molecules on the sliding surfaces. As the protective layer, it not only prevents direct contact between the rough sliding surfaces but also is easy to slide.

Effect of h-BN@Al2O3 on the microstructure and mechanical properties of Si3N4/TiC ceramic composite

Abstract In this study, a new type of (Si3N4/TiC/(h-BN@Al2O3)) ceramic material containing different amounts of coated h-BN were prepared by vacuum hot-pressing sintering. Microstructural analysis showed that the h-BN@Al2O3 particles were dispersed homogeneously and that the particles were tightly bound in the matrix, as determined by scanning electron microscopy. The effects of h-BN@Al2O3 content on the mechanical properties of the composites were also investigated. The optimal content of h-BN@Al2O3 was 10 vol.%. The flexural strength, hardness, and fracture toughness of the material were increased by 22.2 %, 27.0 %, and 5.9 %, respectively, relative to the material to which uncoated h-BN was added.