Cutting conditions of thermal spray coatings
Thermal spray coatings are functional coatings formed on substrate surfaces using thermal spraying techniques (such as flame spraying, arc spraying, and plasma spraying). They exhibit excellent properties such as wear resistance, corrosion resistance, high temperature resistance, and thermal insulation. They are widely used for surface strengthening and repair of mechanical parts, such as turbine blades, rolls, and hydraulic piston rods. Thermal spray coatings are made from a wide variety of materials, including metal coatings (such as nickel-based alloys and aluminum-based alloys), ceramic coatings (such as Al₂O₃ and ZrO₂ ), and metal-ceramic coatings (such as WC-Co and Cr₃C₂-NiCr ). Coatings made from different materials exhibit varying physical and mechanical properties, and their cutting conditions (such as tool material, cutting parameters, and cooling methods) also vary significantly. Properly selecting cutting conditions for thermal spray coatings is key to ensuring machining quality, improving efficiency, and extending tool life.
The selection of tool material is determined by the hardness, wear resistance, and brittleness of the thermal spray coating and is a key step in thermal spray cutting. For metal coatings (hardness 30-50 HRC ), such as nickel-based alloys, which offer superior toughness, carbide tools such as YT726 (suitable for machining high-strength steels and alloys) or YW2 (suitable for machining heat-resistant steels and alloys) can be used. These tools offer high bending strength and wear resistance, can withstand certain impact loads, and are less prone to chipping during cutting. For ceramic coatings (hardness 60-70 HRC ), such as Al₂O₃-ZrO₂ composite coatings, due to their high hardness and brittleness, ceramic or cubic boron nitride ( CBN ) tools are recommended . Al₂O₃ – based ceramic tools (such as AG5 ) have a hardness of up to 93 HRC and are suitable for continuous cutting. Si₃N₄ -based ceramic tools (such as SNMN ) offer superior toughness and are suitable for intermittent cutting. For metal-ceramic coatings (hardness 55-65 HRC ), such as WC-Co coatings, whose wear resistance and hardness lie between those of metals and ceramics, CBN tools are the best choice. For example, the solid CBN tool BN-S30 , with a hardness exceeding 3500 HV , can withstand cutting temperatures exceeding 1200 °C, exhibiting excellent wear resistance and effectively reducing tool wear. Diamond tools, due to their strong affinity with iron-based elements, are only suitable for machining non-ferrous metal-based coatings (such as aluminum) and are not suitable for iron-based or ceramic-based coatings.
Tool geometry optimization must be tailored to the characteristics of thermal spray coatings to reduce cutting forces and tool wear. The selection of the rake angle should take into account the brittleness of the coating. When machining brittle ceramic coatings, a negative rake angle (-5°-0°) is used to enhance cutting edge strength and prevent chipping. When machining tough metal coatings, a smaller positive rake angle (5°-10°) can be used to reduce cutting deformation and friction. A clearance angle of 6°-10° ensures sufficient clearance between the tool and the workpiece, reduces friction between the flank face and the coating, and reduces tool wear. The lead angle should be between 45°-75°. A moderate lead angle balances radial and axial cutting forces, preventing coating flaking due to excessive forces. When machining thin-walled coatings or coatings with low bond strength, a larger lead angle (such as 75°) should be used to reduce the impact of radial forces on the coating. The tool nose radius should be between 0.4-0.8mm. A larger nose radius increases tool nose strength but increases cutting forces. The selected radius should be based on the coating thickness and hardness. For example, when cutting 60HRC WC-Co coating (3mm thickness), the tool uses a -3° rake angle, 8° clearance angle, 60° main deflection angle and 0.6mm nose radius, which can effectively reduce tool wear and coating peeling, and extend tool life by more than 30%.
The selection of cutting parameters must balance machining efficiency and tool life. Different thermal spray coating types require different parameter combinations. Cutting speed is a key parameter affecting cutting temperature and tool wear: When cutting metal coatings, the cutting speed of carbide tools is 80-150 m/min; when cutting ceramic coatings, the cutting speed of ceramic tools is 100-200 m/min, and CBN tools can reach 200-300 m/min; when cutting metal ceramic coatings, the cutting speed of CBN tools is 150-250 m/min. The feed rate should be determined based on the surface quality requirements. For roughing, it should be 0.1-0.2 mm/r, and for finishing, it should be 0.05-0.1 mm/r. Excessive feed rates can lead to coating flaking and increased surface roughness ( Ra ≥ 6.3 μm ), while too low feed rates can increase tool wear due to increased friction between the tool and the coating. The cutting depth should be selected based on the coating thickness, generally 0.1-0.5mm . The initial cutting depth should be 0.1-0.2mm greater than the coating thickness to avoid cutting the tool at the interface between the coating and the substrate (weak interface strength can easily lead to coating peeling). For example, when cutting a 2mm thick Cr₃C₂-NiCr cermet coating, a cutting speed of 200m/min , a feed of 0.1mm/r , and a cutting depth of 0.3mm can achieve a surface roughness of Ra1.6μm and a tool life of 80 minutes.
The cooling, lubrication, and cutting process significantly impact the cutting quality and efficiency of thermal spray coatings and should be selected based on the coating material’s characteristics. Due to the poor thermal conductivity of thermal spray coatings (for example, ceramic coatings have a thermal conductivity of only 1/10-1/20 that of metals), high cutting temperatures are likely to occur during cutting, necessitating effective cooling. When machining metal and metal-ceramic coatings, an extreme-pressure emulsion (10%-15%) can be used, sprayed directly into the cutting zone at high pressure (2-3 MPa) to reduce temperatures and improve lubrication. When machining ceramic coatings, thermal cracking can easily occur due to improper cooling due to the brittleness of ceramics. Therefore, oil mist cooling or air cooling (compressed air) can be used to reduce thermal shock. Regarding the cutting process, a “layered cutting” approach should be employed to avoid cutting to the final dimension in one go. Each layer should be cut to a depth of 0.1-0.3 mm, gradually removing the coating and reducing cutting force fluctuations. For coatings with low bond strength (such as flame-sprayed aluminum), a pre-grooved edge with a depth of 0.5-1 mm should be pre-grooved to prevent overall flaking of the coating during cutting. The surface quality after cutting is generally required to be Ra≤3.2μm. For high-precision parts (such as precision mold coatings), grinding can be used for finishing, but it is necessary to select a soft grinding wheel (such as a white corundum grinding wheel) and a small grinding amount (grinding speed 15-20m/s, feed rate 0.01-0.02mm/r) to avoid overheating of the coating and cracks.
Quality control and tool wear monitoring during the cutting of thermal spray coatings are crucial for ensuring machining quality. For quality control, the coating’s dimensional accuracy and surface roughness must be measured in real time. One part is randomly inspected for every 10 parts processed to ensure dimensional tolerances within IT8-IT10 levels and a surface roughness Ra ≤ 3.2μm. For coatings requiring bonding strength, bonding strength testing (such as the pull-off method) is required. The coating-to-substrate bond strength should be ≥ 30MPa to prevent coating shedding during cutting. Tool wear monitoring can be performed by regularly measuring tool flank wear. When wear reaches 0.3-0.5mm, the tool should be replaced promptly. A cutting force monitoring system can be used to monitor a sudden increase in cutting force of more than 20%, indicating severe tool wear and requiring machine downtime and inspection. Furthermore, chip morphology should be carefully observed during cutting. Normal chips should be continuous ribbons or short coils. The presence of fragmented or lumpy chips indicates improper cutting parameters or tool wear, requiring prompt adjustment. By rationally selecting cutting conditions, optimizing cutting processes and strengthening quality control, efficient and high-quality cutting of thermal spray coatings can be achieved, and the excellent performance of the coatings can be fully utilized.
