Friction And Wear Behaviors Of PVD Nitride Coatings At Elevated Temperatures

As an effective way to improve the properties of material, coating has been widely used in engineering, especially, its excellent anti-friction, corrosion and oxidation resistance abilities, which can improve the wear resistance and life of materials. However, in the working process, the large amount of heat produced by friction can arouse sharp high temperature on the contacting surfaces, therefore the higher requirements are put forward to the high temperature friction and wear properties of the coating materials. In this paper, five nitride coatings were proceed to the further and systematic research, as well as the friction theory, oxidation reaction, high temperature friction and wear behaviors and wear mechanisms around high temperature tribology.TiN, TiAIN, AlTiN, CrN and CrAIN five nitride coatings were developed by Cathodic Arc-evaporation deposition technique, and the properties were detected. The results indicated that the hardness of TiAIN, AlTiN and CrAIN coatings containing Al element were obviously higher than TiN and CrN coatings, and the microstructures of surface and cross-section were compact. The grains of high Al content coatings of AlTiN and CrAIN became smaller. Compared with Ti-based coatings, Cr-based coatings owned smaller surface roughness but higher adhesive strength.Considering thermal stress aroused by the temperature of environment and friction, high temperature friction stress was theoretically analyzed and simulated, and the results indicated that the stress on interface boundaries of coating and substrate changed suddenly. This sudden change was increased with the rise of temperatures, and the coating was fall off easily at the same time. This paper compared the five coatings from the point of view of thermal stress, thermodynamic property of CrAIN coating was the best, and TiN coating was the worst.This paper theoretically analyzed the oxidation properties of the five TiN、TiAlN、 AlTiN、CrN and CrAIN coatings, and then proceeded experimental studies. Thermodynamic calculating of oxidation reaction of the coatings was got using Gibbs-Free Energy function. The results showed that the thermodynamics energetics of Al2O3was the biggest, and Cr2O3was the smallest. From the view point of oxidation kinetics, the oxide films of Al2O3and Cr2O3could maintain integrity and slow-growing, which made it protective. But TiO2was not protective because of the loose structure. The process of coating oxidation was affected interactively by thermodynamics and dynamics factors. With Al, the formation of Al2O3could prevent the deep oxidation of the coating, and with the increase of Al, the anti-oxidation abilities of CrAIN and AlTiN coatings increased. In the same degree, the anti-oxidation abilities of Cr-based coatings were better than Ti-based coatings. The structure of Ti-based coatings presented columnar texture, which made oxygen easy to diffuse to the coating inner along grain boundaries, oxidation was largely controlled by oxygen diffusion to the coating inner. But the inconsistent non-columnar growth of crystal grain made the oxidation of Cr-based coatings slow. The sequence of anti-oxidation ability of the five coatings was CrAlN> AlTiN> TiAlN>CrN>TiN.High temperature friction and wear properties of five PVD nitride coatings were systematic studied between200℃and700℃. TiN and CrN coatings performed not well under high temperature, high speed and high load. TiAIN coating possessed good frictional properties under high temperature, high speed and low load. The friction coefficient and abrasion loss were decreased with the increase of temperature, speed and load, which indicateed that AlTiN coating performed well under high temperature, high speed and high load, but high temperature, low speed and high load were suitable to CrAIN coating. In the same experimental condition, abrasive resistance of CrN coating was lower than that of TiN coating. The addition of Al could increase abrasive resistance of the coating, AlTiN coating with high Al content presented the best properties. This research studied the wear mechanism of the five coatings in high temperature. Because of the friction heat, coating oxidation could easily occur though slight oxidation which could increase friction properties of the coating, but aggravate oxidation could arouse failure. Besides oxidation, the wear mechanisms of TiN coating were abrasive wear, plastic deformation and adhesive wear. Because of the cyclic stress, TiAlN coating got brittle fracture. High content of Al apparently increased the chemical activity of AlTiN coating, which made adhesive wear. For its low hardness, CrN coating easily occured multiple yield, however, CrAIN coating only got abrasive wear and oxidative wear.The influences of hardness on the form of abrasion were studied. There are clear distinctions of abrasive region, wear debris compaction region and wear debris adhesive region on the wear cracks of TiN、TiAlN and CrAIN coatings. Because of debris adhesion, the AlTiN coating presented negative wear. Oxides with different components had great impacts on the wear of coatings, TiO2bring low friction and high wear because of the low hardness and shear strength. Al2O3was protective for coating, which could increase the abrasive resistances of TiAlN and AlTiN coatings along with TiO2. Although mixed Al2O3and Cr2O3oxides film was compact and the bearing capacity was high, but the high hardness could arouse severe wear of grinding ball, abundant derides acting as grains accelerated the wear of CrAIN coating under closed friction environment.