Sintered hard metals

Sintered hard metals are composite materials consisting of hard particles of caibide(s) (only or predominantly WC) bound together by a metalhc binder (only or predominantly cobalt). The carbide content represents 70 to 97% of the total mass of the composite and the average grain size is between 0.3 and 20 pm. 

The present trends in research in the field of sintered hard metals are  

The plasma assisted CVD (PACVD) has had, until now, very little impact, because of the problems of contamination by chlorine of Ti(C,N) or TiN coatings and because of the low ciystalUne nature of alumina coatings. 

The CVD at medium temperature (MTCVD) helps to reduce the decatburization of the substrate. Ti(C,N) coatings could thus be obtained at 800-900°C by using acetonitrile. This MTCVD technique may also be combined very easily with the standard CVD. 

In conclusion, sintered hard metals cover a very wide domain (50%) of machining applications. It is estimated that 70% of turning operations are carried out with carbide tools. A wide range of compositions is offered and each variety is conceived in relation to the requirements of each defined application. Coated carbides also help to reach very high levels of productivity. 

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WC-based cermets are traditionally referred to as sintered hard metals or hard metals . 

As for sintered hard metals, the production of cermets requires an extensive control of the process, from the choice of raw materials to the conditions of sintering (and particularly the sintering atmosphere). Properties of some grades are given in Table 9.1. Finally, quite recerrtly, coating technologies discussed in section 9.2.5 have been extended to cermets. 

While cermets have a hardness similar to sintered hard metals, these are less sensitive to diffusion and they have a better friction and wear behavior. They are capable of machining at speeds higher than those used for sintered hard metals. They excel in the semi-finish turning and milling and finishing of steels and stainless steels. The material properly deficiencies between high-speed steels and sintered hard metals is made up by the composites consisting of a dispersion of TiC or, less... 

Binary grades WC-Co, basically used for applications considered here, represent about a third of the total production of sintered hard metals. 

Figure 9.14 diagrammatically represents different fields of apphcation of sintered hard metals. Other than the field of cutting, already deseribed in section 9.2, and that of drilling being dealt with here, we can see the other fields of applieation which will be discussed in section 9.4. 

The lecoveiy of natural resources from the Earth s crust is an important activity, in which sintered hard metals play a cracial role. It relates to both underground and open cast mining techniques for the recoveiy of minerals - metallic and non-metallic - like carbon (coal), potash, natron, gypsum and of course petroleum drilhng (petroleum, natural gas). The methods of evacuation and the types of tools used depend on the type of stratification encountered. We can classify drilhng into three types ... 

The wear of the tool is the main factor which determines the energy needs and the speed of penetration, based on which the ehoiee of the drilling method and the type of carbide variety for a given type of rock is made. The wear behavior of sintered hard metals is influenced by a certain nirmber of factors the most important ones are ... 

Sintered hard metal (binary grades WC-Co) are used exclusively for making these tools. Cobalt content may vary from 5% to 15% the average grain size of WC may vary from 0.8 to 20 frm (more often 1.5 to 7 (rm) the hardness between 1,100... 

Sintered hard metal tools are now well established in forestry and wood working applications, where they render very high levels of productivity, closer dimensional tolerances, better surface finish and cheaper costs. For example ... 

Diamond is not thermodynamically stable at atmospheric pressure, but its transformation into graphite is extremely slow below 800°C. Its synthesis, achieved for the first time in 1955, takes place by treating graphite at 2,700°C, under 12 GPa pressure, in the presence of a catalyst (1% to 2% of B, Be, Si, etc ), in an equipment (anvil, punches) which besides can be made only in sintered hard metal WC-Co. Present global production reaches 400 million carats, which is more than three times the production of natural diamonds (a carat is 1/5 g). 

PCD (polycrystalline diamond) is obtained by sintering synthetic diamond powder in the presence of a metal binder (Co, Ni or Fe a low percentage by volume), at 1,350-1,500°C imder 5 GPa pressure. One may also sinter a layer of diamonds (0.5 mm thick) on a sintered hard metal substrate, the cobalt of the substrate thus participating in the sintering of the diamond and the adherence of the PCD on the substrate. Inserts up to 72 mm diameter and hardness of 5,000 to 8,000 HV may thus be attained. 

Wear, fatigue and corrosion represent the three plagues of aity mechanical system, because these entail higher maintenance and replacement costs of the stractural components. Due to the special and balanced combination of their properties (compressive resistance, good abrasion resistance, high modulus of elasticity, impact resistance, ability to assume and retain an excellent surface finish), sintered hard metals replace here the majority of other materials and from day to day new applications are discovered. Table 9.2 shows the market share of wear parts in sintered hard metals. 

T able 9.2. Market distribution of wear parts in sintered hard metal... 

High performance ceramics (AI2O3, SiC, Si3N4), pure or reinforced by fiber, have a lower density, a higher chemical inertia and often greater hot hardness than sintered hard metals. However, in the majority of cases, a minimum of toughness must be guaranteed and it is always the sintered hard metal that is finally chosen. 

In applications requiring swing joints of single form, standard size and without any requirements of toughness, silicon carbide can replace sintered hard metals. 

As we have seen in this chapter, on various occasions, the excellent combination of properties on account of their composite nature explains the almost unequalled success of sintered hard metals in a large number of fields of technology. They are still growing in importance despite having been around for 75 years. The appearance of new materials (ceramics, superhard materials) has not dampened the momentum of fundamental and applied research on sintered hard metals. The accent is now placed on technoeconomic gains and the increase in duration of the life of tools, combined with effective methods of recychng. 

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