| Hard coatings deposited by PVD have been used to enhance the ability of high temperature wear-resistant. Chromium nitride is one of the most commonly used coating materials for casting, machining and forming applications because of its high hardness, excellent wear and corrosion resistances. To improve its poor oxidation resistance and tribological performance in high temperature, ternary compounds including other metals such as Al, V, Nb have been explored to prolong the life of die casting tools. In this paper, Al and Nb are added to CrN coatings to form alloy coatings. The different tribological properties of CrAlNbN and CrAIN coatings at high temperature, the details of friction character and mechanism of wear resistant of coatings at various sliding friction stages have been studied. Thermal fatigue property of H13specimens deposited different coatings was tested using the thermal fatigue test machine and crack morphology was observed by optical microscope. The influence of Nb in promoting the thermal bearing capacity has been studied. With the technology of closed field unbalanced magnetron sputter system (CFUBMS), CrAlN and CrAlNbN hard coating were deposited on the surface of the M42HSS and H13steel. The surface and cross-section morphology, the adhesion between the substrate and the coating, phase structure as well as the morphologies for coatings and the worn surfaces was evaluated using the Rockwell indentation, EDX, SEM, GDS and XRD, respectively. A series of pin-on-disc experiments sliding against a WC/Co counterpart ball, were carried out at high temperature(550℃). Effect of alloy elements Nb adding to the CrAlN film showed that:1. Chosen aluminum and niobium as the target materials, CrAlN and CrAlNbN films are prepared using CFUBMS. The coatings deposited at the optimized parameter (target current of2A) is smooth, compact and less defects, exhibiting the gradient composition distribution and the strongly bonding to the substrate. The film grows as columnar crystal with a narrow spacing. The CrAlN coating is composed of CrN and AlN. The CrAlNbN coating contains NbN in addition to CrN and AlN.2. The friction coefficient for high temperature wear increases at initial stage and then decreases gradually. Compared to CrAlN coatings, the CrAlNbN coatings maintain a larger value of friction coefficient, but a lower values for both wear volume and specific wear rate. The fracture toughness of substrates, spalling or fracture of coatings are improved by solution strengthening of NbN precipitate, resulting in a high wear resisitance. The wear mechanism is abrasive for CrAIN coatings and adhesive and oxidation wear for CrAlNbN coatings3. The thermal fatigue properties of H13specimens deposited CrN, CrAIN, CrAlNbN coatings are improved obviously as compared to H13substrates. The thermal fatigue resistance of CrAlNbN coatings is the best because the density and length of cracks on the surface of CrAlNbN coatings is less than those of CrN and CrAIN coatings. It is attributed to the harder compound of NbN which can increase the surface strength of CrAlNbN coatings and restraine the initiation of thermal fatigue cracks. In addition, the oxide of Mb2O5with high temperature stability leads to the improvement of corrosion resistance and is also beneficial to increase thermal fatigue resistance. |
PDF Full Text Request