Enhancement on the transverse fracture strength of functional graded structure cemented carbides

Abstract

Coated cemented carbides have been widely used in machining ferrous and nonferrous metal components, for their advantages in wear resistance, chemical stability, service lifetime and cutting efficiency over traditional uncoated cemented carbides [1,2]. Usually there are mainly two kinds of coating techniques for cemented carbides: physical vapor deposition (PVD) and chemical vapor deposition (CVD) [3, 4]. PVD techniques include sputtering, ion implanting, ion plating and activated reactive evaporation. Comparatively, CVD has been more widely used than PVD for coating cemented carbides, as it provides coatings of higher bonding strength, and hence endows hard alloy with more excellent service properties. However, typical CVD requires reactions at a relatively high temperature (∼1000 ◦C), and leads to microstructure change in the surface of cemented carbides and even bulk distortion problems. The further problems involve possible grain growth and carbon-deficient phase formation in the surface layer close to the coating, deteriorating the bonding strength and stress relaxation between the coating and the substrate [5, 6]. Recently there is an innovation on the coated cemented carbides inserts. A gradient layer lack of brittle cubic phase and enriched in ductile binder phase was formed between the coating and the substrate [6– 9]. This ductile gradient layer could prevent the crack propagation from the brittle coating to the substrate, hence toughens the insert and prolongs the service life. This kind of gradient layer forms by a process called as “gradient-sintering.” Firstly a cemented carbide containing nitrogen is sintered in a usual manner, then it is sintered again in a nitrogen-free atmosphere. During the second step, an outward diffusion of nitrogen occurs. Titanium-rich cubic phases dissolve, and due to strong thermodynamic coupling between nitrogen and titanium, this leads to an inward diffusion of titanium, and a surface zone depleted in cubic phases forms. The thermodynamics and kinetics of the formation of the surface gradient zone have been studied extensively by many materials scientists [6–9]. In this work the transverse fracture strength of the cemented carbides after presintering, gradient sintering and coating were studied, and an enhancement of the transverse fracture strength during the above processes was found for the first time. The cemented carbide alloys studied in this work were based on the mixtures of WC, Ti(C,N) and Co powders. The content of cobalt was 6, 8 and 10 wt% respectively, while that of Ti(C,N) varied from 2.0 to 4.0 wt%. TaC in amount of 6.0 wt% was also added to improve the high temperature wearability of hardalloys. In order to compensate for the carbon loss in sintering and CVD process, about 0.3 wt% carbon powder was added. The powder mixture was milled for 24 hr in a ball mill with cemented carbide milling balls, and alcohol was added to increase milling efficiency. The powders were then dried at about 80 ◦C for 10 hr, and then pressed into 6 mm × 6 mm × 25 mm bars at a pressure of 100 MPa. The compacts were sintered in a vacuum furnace by two steps. Firstly, the compacts were dewaxed at 500 ◦C for 2 hr, and then sintered at 1380 ◦C. The controlled atmosphere was a low-pressure N2 at about 3500 Pa. The gas was introduced when the sintering temperature of 1380 ◦C was reached and was present during the whole process to prevent the evacuation of nitrogen from the compacts. After sintering for 1 hr, the compacts were cooled in the furnace. The first sintering step was called pre-sintering. In order to eliminate surface defects, the samples were ground before subsequent sintering. At the second step, the samples were re-heated to 1420 ◦C and held for 2 hr in a nitrogen free atmosphere, consisting mainly of Ar, in order to develop a composition gradient structure. The second sintering step was called gradient sintering. The sintered parts were then coated with TiN, Al2O3, and Ti(C,N) hard coatings in sequence in a CVD furnace at 1000 ◦C. Before CVD coating, elaborate precautions were taken to ensure the surface to be exceptionally clean and to avoid the discontinuities or diminished adhesion at the interface, which is detrimental to the mechanical properties of the coated materials. The transverse rupture strength was tested on specimens of size 6 mm × 6 mm × 25 mm. The microstructures of polished samples after different stage sintering and coating were observed with a scanning electron microscope (SEM). The SEM was operated in a backscattered mode. Fig. 1 indicates that the transverse fracture strengths of all the cemented carbides were significantly improved after gradient sintering and coating. For example, the WC-6Co added with 2.0 wt% Ti(C,N) had