Friction And Wear Behaviors Of Hard-brittle Tool Materials At Elevated Temperatures

The advanced processing technology, such as dry cutting, high-speed cutting and hard machining, can machine the difficult-to-cut materials with efficient and clean, so it is the important developing direction of the modern machining process. But during the advanced machining process, more cutting heat results in the higher cutting temperature and increased tool wear. Therefore the higher requirements are put forward to the high temperature friction and wear properties of the tool materials. In this paper, the high temperature friction and wear behaviors of three hard-brittle tool materials (PCD tools, Al2O3/TiC ceramic tools and cemented carbide tools) are studied, the wear mechanisms is revealed, so as to supply theoretical foundation for the design and optimization of tool materials.Based on the tribology theory, the change law of the friction coefficient and wear rate at high temperature is analyzed. The Gibbs free energy function is employed to the thermodynamic calculation of the chemical reaction occurred in the components of the PCD, Al2O3/TiC ceramic and cemented carbide tool material. The possible reactions and the oxidation products of each component are analyzed theoretically. The results show that, at high temperatures, the diamond grains in the PCD tool material may be oxidized and graphitized, while the Co binder phase may be oxidized to Co3O4; the TiC in the Al2O3/TiC ceramic tool material may be oxidized to TiO2; the WC, Co and TiC in the cemented carbide tool material may be oxidized to WO3, Co3O4 and TiO2.In this thesis, the high temperature friction and wear properties of three typical hard-brittle tool materials (PCD, Al2O3/TiC ceramic and cemented carbide) are studied.The effects of material composition, microstructure, mechanical properties and test conditions (temperature, speed and load) on the high temperature friction and wear properties are obtained. The results show:the friction coefficient of the PCD tool material increases with the increase of temperature in the range between 200?and 600?, while begin to decrease obviously at 700?. The friction coefficient of Al2O3/TiC ceramic tool material increases with the increase of temperature in the range between 200?and 500?, while begin to decrease from 600?to 800?; the wear rate increases with the elevating of temperature and the order of magnitude is 10-7mm3/(Nm). The friction coefficient of cemented carbide tool material decreases with the increase of temperature in the range between 200?and 600?; the wear rate increases with the elevating of temperature and the order of magnitude is 10-6mm3/(Nm). Among them, the wear rate of cemented carbide with finer grains is about 50% of that with coarse grains under the same conditions, while the addition of TiC and TaC phase can improve the wear resistance of cemented carbide tool material significantly.The three dimensional morphology, cross-sectional profile and surface roughness of the worn surface are studied, and then the track morphology and cross-sectional contour of different tool materials under different conditions are compared. The results show that the wear scar cross-section contour of PCD tool is not obvious, and the depth varies with the surface roughness in the same order of magnitude. The depth of cross-section profile of the wear scar of ceramic tool material increases with the increase of temperature, and the deepest point is up to 1.0?m at 800?. The depth of the wear scar cross-section porfile of cemented carbide tool material increases significantly with the increase of temperature, especially at 600?, in which the deepest point reaches to about 4.0?m.The composition of chemical reaction product on the worn surface and its reaction temperatures are analyzed. It is found that the Co in the PCD tool material is oxidized to Co3O4 at 600?and the graphitization progress of diamond begins at 700?, which is accelerated by the Co and O2. Slightly oxidation phenomenon of Al2O3/TiC ceramic tool material appears at 600?, while the oxidation is obvious at 800?and the oxidation product is TiO2. The cemented carbide tool material starts being slightly oxidized at 500?, while the oxidization at 600?is very obvious and the main oxidation products are WO3 and Co3O4 generated by the WC and Co.Through the comprehensive analysis of the worn surface morphology and surface composition of the brittle tool material, the evolution and wear mechanism of the hard-brittle tool material is explained. PCD tool material is mainly adhesive wear below 600?and the oxidation wear and the graphitization of diamond begins at 600?. Because of the thermal expansion mismatch and the loss of Co, a number of cracks appear between the diamond particles and within them. In the range from 200?to 500?, the wear form of Al2O3/TiC ceramic tool material is pulling out of single crystal followed by large block spalling with the increase of temperature, and the oxidation wear begins at 600?until the oxidation wear is exacerbated at 8008?. The wear mechanism of the cemented carbide tool material below 400?is the precipitation of Co binder phase and the defects and cracks between the WC grains, which lead to the WC particle pulled out and damaged from the worn surface. The oxidation wear appears at part of the cemented carbide tool material at 500?and the oxidation tribofilm made up by the wear debris can be observed at 600?. Grain refinement and the addition of TiC and TaC can increase the wear resistance and oxidation resistance of the cemented carbide tool material. The appearance of oxidation wear increases the wear volume of tool materials. While, because of the low strength and hardness, the oxidation product plays a role in lubricating and reduces the friction coefficient at elevated temperatures.