Frequently Asked Questions
One class of coatings widely prepared using the HVOF spraying process is carbide-containing composites, called cermets. Because of their combination of high hardness, moderate toughness, good adhesion and chemical stability in many service environments, these coatings are widely used to provide protection against wear.
The main constituents are tungsten carbide or chromium carbide particles in a metallic alloy matrix consisting of various combinations of Co, Ni or Cr. The two most common carbide coatings are WC-Co and Cr3C2-NiCr. In the HVOF process, these materials are sprayed as a powder consumable. The coating properties are strongly influenced by the type of HVOF spraying process and the characteristics of the powder.
The powder manufacturing process, which determines the powder characteristics, will therefore influence the properties of the deposited coating. Important powder characteristics that need to be controlled include:
- carbide grain size in the powder particle
- homogeneity of carbide dispersion within the powder particle
- density of the powder particle
- shape of the powder particle
- particle size distribution
For the HVOF spraying process, carbide powder types usually have a particle size in the range 10 to 50µm; a metallic binder content of between 10 to 40 wt%; and within each particle, a carbide grain size of less than 5µm.
All powder manufacturing processes start with an homogenised blend of carbide grains (below 5µm) and a fine powder of the metallic binder (typically 1-2µm in size). Cermet powders suitable for HVOF spraying are prepared from these pre-blended raw materials using either the 'sinter and crushing' process or the 'agglomeration and sintering' process.
Sinter and crushing process
In the sinter and crushing process, the blended raw materials are pressed into bricks or cylinders. These are then sintered at 1100 to 1400°C and subsequently crushed. The crushing stage is performed using steel jaw or hammer crushers. Contamination of the powder with abraded iron dust from the crushers is a particular problem with this process. The iron dust can settle down on the surfaces of the powder particles and is not alloyed with the metallic binder phase. As a result, sintered and crushed powders may produce coatings with less resistance to corrosion than the agglomerated and sintered powders.
Agglomeration and sintering
In this process, the homogenised starting materials are dispersed in water along with a suitable polymer binder (PVA). This dispersion is atomised into a powder by a technique known as spray drying. In this technique, a nozzle or centrifugal atomizer is used to produce droplets of the dispersion inside a heated chamber. The water is rapidly removed and solid particles or pellets are formed, held together by the organic binder. These powder particles or pellets are collected as they fall to the bottom of the chamber.
Sintering is carried out in the same way as the 'sinter and crushing' process but with an initial lower temperature step to remove the organic binder prior to sintering. The spray dried powders may or may not be subsequently crushed. If crushing is required, this can be done with ease because the sintered material is more porous and loosely held together. Use of heavy crushing equipment is therefore not necessary, and iron contamination is avoided. In many cases, the as-spray dried powder has a suitable powder particle size after sintering and does not need to be crushed. These powders can have the advantage of being spherical in shape.
A powder sizing stage is always required on completion of both powder manufacturing processes. Each type of HVOF system and customer application may require a slightly different particle size distribution to achieve best coating properties. Values given by the equipment manufacturers should be used or expert advice sort. A slightly higher powder cost due to tailoring of the particle size distribution can often be absorbed by higher deposit efficiencies and reduced finishing costs.
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