Processing characteristics of thermal spray coatings
Thermal spray coatings are functional coatings formed by heating the spray material to a molten or semi-molten state using a heat source such as flame, arc, or plasma, and then spraying it onto the workpiece surface at high speed. They exhibit excellent properties such as wear resistance, corrosion resistance, and high temperature resistance, and are widely used in mechanical repair and surface enhancement. The processing of thermal spray coatings differs significantly from that of traditional metal materials. Their unique physical and mechanical properties (such as porosity, low bond strength, and unevenness) necessitate specialized techniques and tools. Otherwise, problems such as coating shedding and roughened surfaces can easily occur. A thorough understanding of the processing characteristics of thermal spray coatings is key to ensuring coating performance and processing quality.
The structural characteristics of thermal spray coatings bring many challenges to processing. The primary feature is that the bonding strength between the coating and the substrate is low, and there is a certain amount of porosity inside the coating (usually a porosity of 5%-15%), which can easily cause the coating to peel or crack during processing. The coating is mainly bonded mechanically, with only a small amount of metallurgical bonding. Therefore, under the action of cutting forces, especially when the radial force is too large, the interface between the coating and the substrate is prone to separation. For example, the bonding strength of plasma sprayed nickel-based alloy coatings is about 30-50MPa, which is only 1/10-1/5 of the strength of the substrate material. If the cutting depth is too large or the feed speed is too fast during processing, the local force on the coating will exceed the bonding strength and fall off. In addition, the porous structure of the coating makes the material uniformity poor, and the tool is susceptible to intermittent impact during the cutting process, which aggravates tool wear and vibration and affects the quality of the processed surface.
Thermal spray coatings are typically harder than the base material, and the hardness of different coating types varies significantly, making tool selection difficult. Ceramic coatings (such as Al₂O₃ and ZrO₂ ) can reach hardnesses of HV800-1500 , while metal-ceramic coatings (such as WC-Co ) have hardnesses of HV1000-2000 . However, the base material is often ordinary carbon steel ( HV200-300 ). This hardness gradient forces the tool to adapt to both soft and hard materials during machining, exacerbating uneven tool wear. For example, when machining WC-Co coatings, the coating’s high hardness causes rapid blunting of the tool edge, while the base’s plasticity causes adhesive wear. The tool life of conventional carbide tools is often less than 10 minutes. Furthermore, some coatings (such as self-fluxing alloy coatings) contain hard phase particles, which can cause intense abrasive wear on the tool, further shortening tool life.
Controlling the surface quality of thermal spray coatings is challenging, and defects such as excessive surface roughness and coating burns are common. Because coating materials are often brittle, cutting primarily involves fracture cutting, resulting in powdery or fragmented chips that are difficult to form continuous chips. This leaves more pits and scratches on the machined surface, and the surface roughness is typically one to two grades higher than that of metal. For example, when turning nickel-based alloy coatings, if the tool edge is not sharp or the cutting speed is too low, the surface roughness can reach Ra6.3μm or above, failing to meet sealing or mating requirements. Furthermore, the coating’s poor thermal conductivity (for example, the thermal conductivity of ceramic coatings is only 1/100th that of metals) makes it difficult to dissipate cutting heat, which tends to accumulate in the processing area, causing the coating temperature to rise. When the temperature exceeds the coating’s heat resistance limit, oxidation, phase transformation, or thermal stress cracking of the coating and the substrate can occur. Especially for high-temperature alloy coatings, the cutting temperature must be strictly controlled below 300°C.
The processing method for thermal spray coatings depends on the coating type and workpiece geometry. Common methods include grinding, turning, and honing, with grinding being the predominant. Grinding is well suited to the brittleness and hardness of coatings, achieving high surface quality through micro-edge cutting with the grinding wheel. Common grinding wheel types include resin-bonded diamond wheels (suitable for ceramic coatings) and cubic boron nitride (CBN) wheels (suitable for metal-based coatings). Grinding parameters must be strictly controlled, with a grinding wheel speed of 15-25 m/s, a feed rate of 0.01-0.03 mm/r, and a grinding depth of no more than 0.05 mm. A grinding fluid with excellent cooling properties (such as synthetic grinding fluid) should be used to reduce grinding heat. Machining is suitable for roughing or semi-finishing large coatings. Specialized tools (such as PCBN tools) are required, with cutting speeds of 100-200 m/min, feeds of 0.05-0.1 mm/r, and a cutting depth of 0.1-0.3 mm to avoid excessive cutting forces. Honing is used for precision machining of coatings. It improves surface quality through low-speed grinding with oilstone. It is suitable for key parts such as sealing surfaces and bearing holes. The surface roughness after honing can reach Ra0.1-0.4μm.
Key process control points for thermal spray coating machining include tool wear monitoring, cooling and lubrication, and machining allowance control. Because tool wear is rapid during coating machining, a real-time monitoring mechanism is necessary. Cutting force sensors or acoustic emission sensors are used to assess tool status. Tool wear should be monitored promptly when tool wear reaches 0.2mm to prevent coating damage caused by excessive tool wear. The cooling and lubrication system should utilize a high-pressure, high-flow design. The cooling nozzles should be positioned close to the cutting area to ensure that the cutting fluid effectively envelops the tool and workpiece. For heat-resistant materials such as ceramic coatings, oil mist lubrication can be used to reduce coolant usage and improve lubrication effectiveness. Machining allowances should be appropriately allocated. Generally, coating thicknesses range from 0.3-2mm, with 60%-70% removed during roughing and 0.1-0.2mm remaining during finishing to ensure surface integrity. Furthermore, machining equipment must be sufficiently rigid. Spindle runout on lathes or grinders should be controlled within 0.01mm to prevent equipment vibration that exacerbates coating cracking and flaking. Through targeted process control, the excellent performance of the thermal spray coating can be fully utilized and the service life of the workpiece can be extended.
