Study On Wear Properties Of Plasma Nitrided Layers And CrN-based Composite Films On H13 Steel

AISI H13 steel is an important ferrous alloy which is one of most widely used and most representative hot work die steel. However, during working conditions, the surface of the mold cavity sustains a cyclic impacting load with materials processed in the state of high temperature which lower the lifetime of die. The main failure forms of H13 steel include thermal corrosion, thermal erosion wear and thermal fatigue in service. Surface treatment technique can be used to effectively prolong its service life.The H13 steel was nitrided and coated with CrN, CrAIN, CrAlSiN films using a plasma nitriding technique and multi-arc ion plating technique. The micro structure and composition were characterized with Buehler automated image analyser, X-ray diffraction(XRD) and field emission scanning electron microscopy (FESEM). The mechanical properties, such as room temperature and high temperature wear resistance, microhardness and the adhesion strength were measured with pin-on-disc wear test machine, HT-2001 abrasive wear test machine, Leco M-400-H1 microhardness tester and rockwell apparatus. The nitrided layers with different thicknesses and components were obtained by changing nitriding pressure, temperature, time and N2 proportions. Results show that there is only diffusion layer formed on the surface of H13 steel at the pressure of 200 Pa,300 Pa, and at the the nitriding temperature of 540℃. Other nitrided layers consist of compound layer and diffusion layer. XRD analysis shows that nitrided layers are mainly composed ofε-Fe2-3N,γ-Fe4N, a-Fe, Fe2O3 and Fe3O4 phase. The surface hardness of plasma nitrided H13 steel has doubled compared with substrate. The results of room temperature wear test indicate that the friction coefficient of the nitrided H13 steel are reduced at the pressure of 200Pa and 300Pa, and the wear rate decreases triply compared with substrate. High temperature wear test shows the nitrided H13 steel at the pressure of 100Pa has lower friction coefficient, and the wear rate of it is reduced more than 6 times compared with substrate.CrN films deposited with a bias voltage of 300V and arc current of 60A exhibit smooth surface and dense structure columnar crystal, and the surface hardness increases more than 3 times compared with substrate. The bonding strength between films and substrate reachs HF3. Comparing with substrate, CrN film shows good high temperature wear resistance due to its lower friction coefficient and wear rate. CrAIN films with different thicknesses and structure were obtained by changing Cr arc current and bias voltage. The bonding strength between films and substrate is low owing to high hardness and internal stress. The Cr content increases and the Al content decreases with the increase of Cr arc current. The CrAlN films exhibit high density when the bias voltage is 300V and 500V. The high temperature wear resistance is the best when the Cr arc current is 45A whose wear rate is half of substrate.Nitrided and non-nitrided CrAlSiN films deposited with different Cr and AlSi arc current exhibit rough surface and hierarchy cross section. Nitrided CrAlSiN film shows high bonding strength (HF1) between films and substrate compared with non-nitrided CrAlSiN films (HF3-HF4). Both of nitrided and non-nitrided CrAlSiN films obtained at ICr:IAlSi=1:1 shows good high temperature wear resistance with half wear rate compared with substrate.