The coating of objects to enhance or add characteristics is a practice that is widely used. There are many different types of coating and numerous different techniques for applying them. One of the main ways to apply a coating is by using plasma spray.
This method involves propelling molten materials at high velocity onto the object/item/surface (usually referred to as the substrate). This is one of five methods categorised as ‘thermal spray’ coating. The others are combustion flame spraying, vacuum plasma spraying, high-velocity oxy-fuel spraying (HVOF), and two-wire electric arc spraying.
The plasma spray process
There are four main stages to the plasma spray coating process:
Stage 1: Preheating
This step involves heating the substrate prior to the coating being applied. The temperature and duration of this stage are carefully controlled so that the adhesion and quality of the coating are optimised. This makes the substrate more receptive to the coating and enhances the strength of the bond.
Stage 2: Powder injection and melting
Firstly, the fine powder particles are injected into the extremely hot plasma jet where they melt and turn into molten droplets. These droplets are then propelled onto the substrate, solidifying on impact and beginning to form the coating layer.
Stage 3: Coating build-up
At this stage, the coating layer slowly builds up. This can be carefully and precisely controlled to ensure the requisite thickness and uniformity across the substrate’s surface. This also enables specific design/performance requirements to be met.
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Stage 4: Cooling and solidification
The droplets that are applied to the substrate then cool and solidify. This part of the process is important as it determines the microstructure and key properties of the coating. Proper cooling prevents defects such as cracks and also promotes strong adhesion.
Plasma spray coating has many advantages and is used to coat a wide range of items across industries.