AlCrN coating was prepared on the surface of TiC ceramic cutting tool by cathodic ion coating method. The friction and wear behavior of the coating under different loads at 900°C were investigated by ball/plane contact method. The wear trace profile and microscopic morphology were observed by scanning electron microscopy. The changes of chemical elements and phases on the surface of the coating after wearing were analyzed by energy scattering spectroscopy and X-ray diffraction. The results show that after oxidation at 900°C, all N elements in the coating are released to form oxides of Al and Cr, which improves the lubricating and wear properties. The average friction coefficients of the coating were 0.1455, 0.3939, and 0.4188 under the load of 600, 800 and 1000g. It showed excellent friction properties when loaded with 600g and it is suitable for precision machining. At high temperatures, AlCrN coatings exhibit oxidative wear, accompanied by a small amount of abrasive wear and adhesive wear.
With the development of high-speed, high-precision and efficient dry cutting, surface coating technology is the main way to effectively improve tool performance. CrN coating has the advantages of high hardness, high wear resistance and low friction factor. It is widely used in tool surface modification. However, the working temperature of CrN coating is only 650°C, which is not suitable for high-temperature machining. The CrN crystal is a face-centered cubic structure. After adding Al atoms, some Cr atoms in CrN are replaced, and Al atoms are dissolved into CrN crystals. Then CrN crystal structure changes from face-centered to hexagonal structure, and its microstructure, mechanical properties and wear properties were affected significantly.
Two kinds of compact oxides Cr2O3 and Al2O3 were formed in the high temperature to enhance its thermal stability, the anti-oxidation temperature of it can reach 900 °C, and it can still maintain high hardness, high wear resistance, high temperature oxidation resistance and adhesion to the substrate for good performance, we believes that it is suitable for efficient dry cutting of very large ring gears. The authors used cathode arc ion coating method to prepare AlCrN coatings on the surface of TiC cermets and analyze its friction and wear behavior at 900°C, which provided a technical reference for efficient machining of super-large ring gears.
The base material is a TiC-based ceramic cutting tool, which is made by sintering nanometer-scale TiN mixed with micro-sized TiC, wherein the hard phase is TiC and TiN, and the adhesive is Ni. Its chemical composition (mass ratio) is Ti51.26 %, W19.55%, C12.92%, Ni7.63%, Co8.64%. After degreasing and sandblasting, the sample was ultrasonically cleaned with acetone solution and dehydrated with anhydrous ethanol. After drying in a constant temperature oven, it was coated on a PVT coating machine. Using 99.99% purity of Cr and Al as targets, the coating parameters: vacuum degree 3×10-3 Pa, furnace temperature 500°C, reaction gas N2, coating time 120 min. Using N2 gas as protection, after annealing at 180 °C for 2 hours, using acetone in the KQ2200DE type NC ultrasonic cleaning, and then ultrasonic cleaning with deionized water, and finally dried with a hair dryer to obtain the required sample. The friction-wear characteristics of the AlCrN coating at 900°C were investigated by a HT-1000 high-temperature friction and wear tester. The test parameters were: load of 600, 800, and 1000g respectively, the temperature was 900°C and it was adjusted by a 30-stage programmable temperature controller. Accuracy is 0.2%FS (full scale), ceramic ball is used for abrading parts and friction radius is 3mm, rotation speed is 1000r/min. After the wear test, the surface morphology of the coating before and after high temperature wear was observed with a SUPRA55 scanning electron microscope. The chemical composition and phase change of the coating before and after high temperature wear was analyzed by a scanning electron microscope (EDS) and a D/max2500PC X-ray diffraction (XRD) instrument to study the wear failure mechanism of the AlCrN coating under high temperature.
Analysis And Discussion Of Results
Fig. 1(a) shows the surface morphology of AlCrN coating at room temperature. The particles on the surface are relatively small. The reason is that the sputtering yield of Al target increases and the coating nucleation rate increases accordingly. The surface of the coating is relatively smooth, and there are many pits of various sizes, which due to the reverse sputtering effect of the coating surface caused by ion bombardment. And to a certain extent, the surface roughness of the coating is reduced. AlCrN coating chemical elements mass fraction: Al36.72%, Cr36.11%, N27.18%; atomic fraction: Al34.06%, Cr17.38%, N48.56%, as shown in Figure 1 (b) below. AlCrN coating composition is Al, Cr, and N three elements, the ratio of the number of atoms close to 2:1:3, and that shows that the coating is mainly composed of nitrides of Al and Cr, which is beneficial for improving the hardness and oxidation resistance of the coating.
Fig.1 Surface morphology and EDS analysis of AlCrN coating
(1) After oxidation at 900°C, all N elements in the coating are released and Al2O3 and Cr2O3 are formed on the surface. Among them, Al2O3 oxide has a friction-reducing effect in the wear process. Cr2O3 improves the layer hardness and wear properties of the coating.
(2) Under the action of 600, 800 and 1000g load, the average friction coefficient of the coating is 0.1455, 0.3939, and 0.4188 respectively. Among them, excellent friction characteristics are exhibited under a load of 600g, which is suitable for precision machining.
(3) In the friction process at 900°C, a large amount of oxides are produced in the wear scars of the coating, which is caused by the diffusion of the matrix atoms at high temperatures, and it is characterized by oxidation wear, followed by a small amount of abrasive wear and adhesive wear.