ADVANCED MATERIALS & PROCESSES | APRIL 2024 34 iTSSe TSS iTSSe TSS FEATURE Fig. 1 — Mini-Nova inside a 3-inch cylinder. 5 are referenced in Table 2 for a WC-10Co4Cr coating. ID-HVOF coatings have applications in various industries, including oil and gas, chemical processing, power generation, and aerospace, the latter for both turbine components and landing gear. By applying a protective coating to internal surfaces, components have improved performance, reduced maintenance, and longer service life. The Mini-Nova HVOF represents the latest breakthrough for HVOF coating devices allowing coatings to be applied to internal diameters below 3 inches (76 mm) (Fig. 1). This patented technology facilitates high quality coatings that meet industrial standards set for traditional HVOF torches. Using spray distances between 20-40 mm, hardnesses exceeding 1000 HV0.3 and porosities below 1% are achieved for WC-10Co4Cr coatings, and these are sprayed with deposition efficiencies exceeding 65%. The Mini-Nova is shown in Fig. 2 uses fine 5-15 µm powder and produces coatings with particle velocities exceeding 1000 m/s with temperatures reaching 2000°C as measured using Tecnar Accuraspray. ID-NOVA WARM SPRAY The Mini-Nova is also adaptable for warm spray (WS) applications to further optimize parameters with inert gas or nitrogen additions. Warm spray is the addition of inert gas into the fuel stream to increase gas velocity and reduce the flame temperature. This is a useful HVOF tool to optimize coating properties and extends the range of coating materials that can be sprayed. Kuroda et al.[1] pioneered the introduction of warm spray for the application of titanium coatings for corrosion protection in marine splash zones. The ID-Nova uses warm spray to increase coating density by increasing the velocity of impacting particles. K2 ULTRA-HIGH VELOCITY OXYGEN FUEL SPRAYING Ultra-high velocity oxygen fuel (UHVOF) spraying represents an alternative to HVOF warm spray[2]. Faster and cooler particles are achieved using combustion chambers with smaller critical diameters as seen in Fig. 3. Conventional combustion chambers (7.8 mm critical diameter) result in combustion chamber pressures between 0.8-1.0 MPa whereas combustion chambers with 5.0 mm critical diameter have a combustion chamber pressure up to 1.8 MPa approximately double that of the conventional set-up. This results in a cooler, faster spray jet and enables low oxidation aluminum alloy particles to impact substrate surfaces in mostly the solid state and copper coatings produced with electrical conductivity like cold gas sprayed coatings. UHVOF also facilitates the spraying of fine WC-10Co4Cr powders with 2-10 µm particle size without barrel loading to produce a sprayed coating with surface roughness with a Ra < 2 µm. TABLE 2 — WC-10Co4Cr HVOF COATING REQUIREMENTS Coating property Value Porosity < 1% Hardness > 900 HV0.3 Bond strength > 69 MPa (10,000 psi) Stress state Compressive Fig. 2 — Mini-Nova before spraying on the internal surface of a cylinder. Fig. 3 — K2 cross section showing the combustion chamber’s critical diameter.
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