Tungsten carbide materials are generally recommended for wear applications with service temperatures of less than 500° C (930° F) as higher temperatures will result in the formation of brittle phases that reduce wear resistance and coating integrity. Chemistry, manufacturing process, individual carbide size and spray process are critical to overall performance, as is application tribology. Typical wear applications include erosion (low angle), abrasion, fretting, sliding wear and impact resistance. Matrixes of higher colbalt levels improve coating toughness. The addition of chromium improves impact resistance. The addition of chromium improves atmospheric corrosion. Powder selection and spray process is important for applications with specific surface finish requirements, such as smooth as-sprayed surfaces, fine ground and finished surfaces or super finishes. Coatings applied using the HVOF process are dense and well-bonded, with a more homogeneous structure than can be obtained using air plasma or combustion powder spray processes.
Tungsten Carbide Coatings deliver superior value and performance as compared to hard chrome plate not to mention environmentally friendlier. They provide superior wear prevention and corrosion protection performance. With Improved life performance over time this coating can be the more cost-effective solution.
| PHYSICAL COMPARISON | TUNGSTEN-CARBIDE | CHROME | |
| Macrohardness (Rc) | > 70 | 60-70 | |
| Microhardness (DPH 300) | > 1050 | 750-850 | |
| Bond strength *Results exceed strength limit of epoxy needed for tensile test |
> 10,000* | 6,000 | |
| Porosity | < 1% | Inherently cracked | |
| >Coating thickness typical (in) | .003-.012 | less than .005 | |
| Surface finish (Ra) | < 4 | < 4 | |
| Corrosion Test – ASTM B117 (hours) | 720 | 55 | |
| Surface temperature limits (F) | 1025 | 750 |
In this process the powder is deposited as a low velocity coating and then remelted at 2000 deg. F. The bonding is metallurgical and the compressive strength is very high with almost zero porosity. Used for resistance to abrasion where some impact or loading may occur. The drawbacks are the high temperatures involved to fuse the coating. Large components can now be fused using a laser but this equipment is very expensive. The consumable is usually tungsten-carbide mixed with nickel or cobalt matrix. Only the matrix actually fuses to the substrate. Common powders are Metco 36C, Eutectic 23075, Colmonoy 705.
In this process ionized gas is used to heat and propel the tungsten-carbide at moderate speeds ( typically Mach I ) Operating costs are lower than HVOF and the bond strengths and overall densities are reduced. Higher cobalt content (17%) is used to create a tougher coating with some flexibility. Hardness is slightly reduced due to reactance in the heat of the plasma. This is often the most economical process to deposit Tungsten-Carbide. Common powers are Metco 73F, Eutectic CPP 2506, and Praxair AI 1173.
In this process the powder is accelerated to trans-sonic speeds by the rapid combustion of fuel and large quantities of oxygen. The coating is extremely dense and the carbides are left unreacted by the heat and extremely hard. This is the hardest of the carbide coatings and will take moderate loads or impact. The system has high operating costs but HVOF is finding acceptance in roll facing and chrome plating replacement. The consumable is usually tungsten-carbide in a 12% cobalt matrix. Common powders are Metco Diamalloy 2005, Eutectic 19910, Praxair WC 489.