Study On Properties Of Nitriding/TiAlN Duplex Coatings On Hot Work Steel

H13as a common steel for alumium die-casting mold is subjected to alternate heating and cooling, high mechanical loads and molten aluminum touching in practice, which will cause some failures such as thermal fatigue, brittle fracture, deformation, wear and surface corrosion. Since the failure of dies generally starts form the mold surface, various surface treatment techniques have been developed to extend H13die life.In this paper, TiAIN coatings were deposited on plasma-nitrided H13substrate to form duplex TiAIN coatings on it using plasma nitriding and closed field unbalanced magnetron sputter ion plating. Single TiAIN coatings were also prepared on non-nitrided H13steel on contrasting analysis. The influence of the nitrided layer with different surface treatments on the load carrying capacity and microstructure of composite coatings was studied. The thickness, surface and cross-sectional morphology of the TiAIN coatings were observed using scanning electron microscopy and optical microscopy. EDS and XRD analysis were used to detect the composition and phase structure of TiA1N coatings, respectively. The load-carrying capacity was synthetically evaluated by means of Rockwell-C indentation and scratch tests. Thermal fatigue performance of H13steel samples with single TiA1N or duplex TiA1N coating was carried out by using self-design thermal fatigue test device. Thermal crack morphology after thermal fatigue test was observed using optical and scanning electron microscopy, and substrate hardness before and after thermal fatigue test was measured using Vickers micro-hardness measurement. Rockwell-C indentation was used to observe indentation edge crack morphology. Wear tests were performed on WC-Co ball-on-disc wear device in order to study the wear mechanism, wear rate and friction of TiA1N coatings. The surface morphology and composition of wear tracks were analyzed by SEM and EDS. The results are showed as follow:1. Surface polishing of nitrided H13steel is beneficial to compact coatings growing on the polished substrate and the loose layer obtained after nitriding can cause more surface defects, exposure and uneven distribution of columnar crystals. The high surface-hardness of the nitrided layer and its smooth hardness gradient can effectively prevent coating cracking and spalling. TiA1N coatings formed on the nitrided substrate without any surface treatment are susceptible to spalling, showing a lower scratch resistance.2. Thermal fatigue crack initiation and growth is actually the process of plastic deformation. Localized defects of coating surface can lead to the thermal stress concentration, which makes crack much easier to initiate. The thermal fatigue crack initiation and propagation are influenced by hardness and toughness of substrates. Therefore, removing the compound layer in the nitrided layer as well as polishing nitrided surface can reduce the duplex coating surface defects, and effectively restrain crack initiation in the hard duplex coating. And the existence of nitrided layer can inhibit the surface softing of substrate and prevent molds from cracking or spalling under high loads.3. The wear rate of duplex TiAIN coating is significantly lower than that of single TiAIN coating on H13steel due to the strong support of nitrided H13substrate. Since non-nitrided H13steel substrate is easy to produce plastic deformation, the TiAIN coating formed on H13surface is worn breakthrough the coating in the early sliding, causing the spallation of coatings in the wear track centre to be pushed to the track edge. It results in the different wear mechanisms between the centre and edge of the ball and irregular oval shape on the worn surface of balls.