Cermet compositions, such as cemented tungsten carbide (WC—Co), are well known for their mechanical properties of hardness, toughness and wear resistance, making them a popular material of choice for use in such industrial applications as mining and drilling where its mechanical properties are highly desired. Because of its desired properties, WC—Co in, particular has been the dominant material used as hard facing, wear inserts, and cutting inserts in rotary cone rock bits. The mechanical properties associated with WC—Co and other cermets, especially the unique combination of hardness toughness and wear resistance, make these materials more desirable than either metals or ceramics alone.
Although WC—Co is known to have desired properties of hardness, toughness and wear resistance, it is also known to suffer from thermal shock-related fatigue cracking in many applications. For example, WC—Co compacts that are used as cutting elements for drill bits often develop a cris-cross pattern of cracks in wear flat surfaces or “wear flats”. The pattern of cracks formed on these wear flats is known as “heat checking” and is caused from exposing the wear surface to cyclic abrasive friction heat and drilling fluid cooling during the drilling operation, e.g., when the drilling assembly is rotated. Such heat checking is known to be the cause of thermal shock or thermal fatigue related crack formation, crack propagation, and ultimately catastrophic failure.
The problem of heat checking is attributed to the relatively poor thermal properties of the cobalt (the binder material) when compared to that of the tungsten carbide. Prior attempts to correct this problem, to increase the heat checking resistance of WC—Co, has been to reduce the cobalt binder content and balance other mechanical properties of the composition through grain size adjustment. However, reducing the cobalt binder content adversely impacts other properties of the resulting cemented tungsten carbide material. Generally speaking, as you decrease the cobalt binder content you also reduce the fracture toughness, and increase the hardness, of the cemented tungsten carbide, thereby making the composition brittle and more susceptible to fracture and failure. As you increase hardness you also increase wear resistance, but this is all at the expense of fracture toughness.
Fracture toughness is a limiting factor in demanding industrial applications such as high penetration drilling, where WC—Co inserts often exhibit gross brittle fracture that leads to catastrophic failure. Thus, prior attempts at addressing unwanted heat checking, to reduce or control thermal shock related catastrophic failure, has been at the expense of reduced fracture toughness, which also is known to cause catastrophic failure.
It is, therefore, desirable that a cermet composition be developed that has improved thermal shock resistance when compared to conventional cemented tungsten carbide materials. It is desirable that such cermet composition display improved thermal shock resistance without sacrificing such properties as fracture toughness and wear resistance when compared to conventional cemented tungsten carbide materials. It is desired that cermet compositions of this invention be adapted for use in such applications as rock bits, hammer bits, mining and drill bits, and other applications such as mining and construction tools where improved thermal shock resistance is desired.