Carbide formers

Another approach is to coat the cutting tool material with a carbide former, such as titanium or siUcon or their respective carbides by CVD and deposit diamond on top of it. The carbide layer may serve as an iaterface between diamond and the cemented carbide, thus promoting good bonding. Yet another method to obtain adherent diamond coatings is laser-iaduced microwave CVD. By ablating the surface of the substrate with a laser (typically, ArF excimer laser) and coating this surface with diamond by microwave CVD, it is possible to improve the adhesion between the tool and the substrate. Partial success has been achieved ia this direction by many of these techniques. 

Use materials that contain strong carbide-formers, the stabilizers. 

The carbide precipitation that leads to sensitised grain boundary regions can be minimised by reducing the carbon to 0.03% or less but this increases the cost and reduces the strength. The alternative is to add stabilising elements such as titanium or niobium, which are stronger carbide formers than chromium. There are numerous texts that describe the metallurgy of... 

Chemical Resistance. C-BN is essentially inert to all reagents at room temperature. It does not react with carbide formers such as Fe, Co, Ni, Al, Ta, and B at approximately 1000°C (while diamond does) this is a useful characteristic in machining and grinding applications. However, it reacts with aluminum at 1050°C and with Fe and Ni alloys containing Al above 1250°C.P4]... 

It is clear that, in order to protect alloys containing chromium from oxidation, atmospheres based on the CO2-CO system cannot be used if carburization is to be avoided. Here, and in other cases where strong carbide formers are involved, protection may be achieved by using the H2—H2O system for which the relevant... 

Refractory metals are used as carbide formers (vanadium) in alloys that contain insufficient chromium to form a protective layer of Cr203 (M0O3 or WO3 in Mo or W containing alloys) or as solution strengthening elements in Co-based alloys (Mo or W) [8]. 

In carbon steels, Cr is added to increase corrosion and oxidation resistance because it promotes the formation of stable passivating and protective oxide layers. Moreover, Cr is a strong carbide former which modifies and delays the formation of pearlite and bai-nite, thus increasing the hardenability. In heat-resistant steels Cr contributes to the high-temperature yield strength. 

The transformation diamond-graphite is also a function of the environment. It becomes especially rapid in the presence of carbide formers or carbon soluble metals. For instance, in the presence of cobalt, the transformation can occur as low as 500°C. However, in hydrogen diamond is stable up to 2000°C and in a high vacuum up to 1700°C. 

Substrate Materials. Cemented tungsten carbide (WQ is a major substrate cutting tool material. It has been successfully coated with CVD diamond with good adhesion especially with an intermediate carbide-former layer.l JI ... 

Titanium is a strong carbide former and is used as an alloying element in stainless steels, containing some carbon. Titanium binds carbon and decreases the negative effect of carbon on corrosion resistance after welding. In steel manufacture, titanium is added to the melt as ferrotitanium. 

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