Oxygen Pressure Fine-Tunes Zirconium Carbonitride Coatings for Extreme Wear Resistance

Tool steel components that endure contact loads, abrasive and adhesive wear, and cyclic friction live short lives unless their surfaces are protected. In a new study published in Frontiers in Chemical Engineering, researchers from Kazakhstan and Russia report that a single, easily adjustable dial in the high-velocity oxygen fuel (HVOF) spraying process — the pressure of the oxygen fed into the combustion chamber — can make the difference between a mediocre wear-resistant coating and one that cuts friction by nearly forty percent and multiplies hardness more than fivefold. The material in question is zirconium carbonitride, Zr2CN, a nitride-carbide ceramic prized for its hardness, thermal stability and resistance to seizing under friction.

Zirconium carbonitride belongs to a family of materials whose chemistry sits deliberately between two extremes. Both zirconium carbide and zirconium nitride crystallize in the same rock-salt-type face-centered cubic structure, so carbon and nitrogen atoms can substitute for one another in a continuous solid solution. By tuning the carbon-to-nitrogen ratio, engineers can steer the balance of mechanical and tribological properties in ways that binary carbides or nitrides alone cannot offer. Until now, however, producing Zr2CN coatings on steel has largely meant vacuum-based PVD or CVD methods, which impose strict surface preparation requirements, limit coating thickness and part geometry, and constrain productivity. HVOF spraying, by contrast, propels powder particles at roughly 1,500 meters per second through a flame of about 3,000 degrees Celsius, building dense, low-porosity, minimally oxidized coatings at industrial throughput.

The team, led by Mazhyn Skakov and Sardor Kambarbekov of Sarsen Amanzholov East Kazakhstan University, sprayed Zr2CN powder onto plates of U8G carbon tool steel using a Termika-3 HVOF system. The powder, synthesized to technical specifications, showed a narrow and symmetric particle size distribution centered around 25 to 30 micrometers, with irregular polyhedral particles in the 10 to 100 micrometer range. Sandblasting the steel with white alumina grit before deposition improved adhesion. Crucially, the researchers held the spraying distance at 0.3 meters, the propane pressure at 2 × 10^5 Pa and the air pressure at 3.2 × 10^5 Pa, varying only the oxygen pressure across three settings: 4.0 × 10^5, 4.3 × 10^5 and 4.5 × 10^5 Pa.

X-ray diffraction revealed that every coating was a multiphase system containing the Zr2CN carbonitride, zirconium carbide, zirconium nitride, zirconium oxide and alpha-iron. The carbonitride’s lattice parameter, refined at approximately 4.636 angstroms, falls neatly between the reference values for ZrC (4.698 angstroms) and ZrN (4.5675 angstroms), and a Vegard’s law calculation yielded a carbon-to-nitrogen ratio close to unity. The presence of carbide and nitride phases points to partial decomposition of the original powder under the flame’s extreme heat, while weak oxide peaks trace to oxidation of particles during their flight through the combustion gases. The strong alpha-iron reflection, most pronounced in the thinnest coating, signals substrate contribution to the diffraction pattern rather than a coating constituent.

Rietveld refinement quantified these trends. The coating sprayed at 4.3 × 10^5 Pa contained the highest Zr2CN fraction at 23.4 weight percent and the lowest ZrO2 fraction at 11.2 weight percent. The lowest-pressure coating showed the highest alpha-iron contribution at 37.8 weight percent, confirming a significant substrate signal arising from its reduced thickness. The authors caution that because Zr2CN, ZrC and ZrN reflections partially overlap, these refined fractions should be treated as semi-quantitative estimates, interpreted alongside peak-position analysis and elemental mapping rather than as definitive phase purities.

Williamson-Hall analysis of diffraction-line broadening added a nanoscale dimension to the picture. The intermediate oxygen pressure produced the largest coherent scattering domain size, approximately 82 nanometers, and the lowest microstrain, about 0.30 percent, indicating more favorable crystallization conditions and reduced internal stress. At the highest oxygen pressure, microstrain rose to roughly 0.51 percent and crystallite size shrank to about 16 nanometers, consistent with intensified oxidation and partial degradation of the carbonitride phase. At the lowest pressure, microstrain peaked near 0.59 percent with the smallest domain size, reflecting incomplete coating formation. The researchers note these values are comparative estimates, given the overlap of the relevant reflections.

Scanning electron microscopy of cross-sections told a matching story. The coating deposited at 4.3 × 10^5 Pa was the thickest at 13.75 ± 2.00 micrometers, dense and uniform, with a clearly defined interface to the substrate and the lowest apparent cross-sectional porosity of 0.31 percent, measured by threshold-based image analysis in ImageJ. The lowest-pressure coating, at 8.75 ± 1.25 micrometers, showed pronounced porosity and structural non-uniformity near the interface, likely from particle over-oxidation and partial decomposition in flight. The highest-pressure coating, at 11.33 ± 1.50 micrometers, contained isolated pores and localized inhomogeneities suggestive of insufficient particle kinetic energy at impact. Elemental mapping by energy-dispersive spectroscopy showed the intermediate-pressure coating had the most homogeneous distribution of zirconium, carbon and nitrogen with minimal oxygen, while its counterparts displayed oxygen enrichment in the upper layer and weakened carbon and nitrogen signals — fingerprints of carbonitride decomposition into oxide compounds.

The mechanical and tribological results converged on the same conclusion. The optimal coating achieved a microhardness of 1826 ± 38.4 HV0.5, more than five times the 304.9 HV of the untreated steel, along with the highest apparent pull-off adhesion strength of 18.4 MPa and the lowest, most stable friction coefficient of approximately 0.54 ± 0.01 against a 100Cr6 steel ball under a 5-newton load in dry sliding. Ball-on-disc testing per ASTM G99 quantified the payoff: the lowest wear volume of 12.33 × 10^-3 cubic millimeters and a specific wear rate of 0.82 × 10^-4 cubic millimeters per newton-meter, about 30 percent lower than the lowest-pressure coating and 21 percent lower than the highest-pressure one.

Wear-track microscopy revealed how oxygen pressure shifted the dominant wear mechanism. The lowest-pressure coating, porous and chemically heterogeneous, suffered deep grooves, macro-cracks and debris pits characteristic of abrasive wear, its microstructure unable to support the cyclic contact load. The optimal coating showed only shallow, fine abrasion grooves and small oxidized wear particles, with no macro-cracks or spalling — evidence of mild adhesive wear in which a thin tribofilm of oxidized particles may act as a lubricating layer. The highest-pressure coating displayed surface cracks, large spalling pits and localized delamination, consistent with oxidative and delamination wear driven by its brittle, oxide-rich surface. The authors emphasize that direct post-wear chemical analysis of the tribofilm, such as Raman spectroscopy or point EDS on the wear debris, was not performed and would be needed to confirm these mechanism assignments definitively.

The broader significance lies in the simplicity of the control knob. Because HVOF is a fiercely non-equilibrium process — particles experience milliseconds of heating, acceleration, impact and rapid solidification — the final phase composition cannot be predicted from thermodynamics alone. Yet the study demonstrates that a modest adjustment of oxygen pressure, from 4.0 to 4.5 × 10^5 Pa, systematically reshapes phase retention, crystallite size, porosity, hardness, adhesion and friction behavior. The intermediate setting of 4.3 × 10^5 Pa balanced particle heating, velocity and limited oxidation to produce the densest, most crystalline and best-performing coating. For industries seeking to extend the service life of carbon steel tooling without vacuum chambers or geometric constraints, the finding offers a practical recipe: tune the oxygen, and the carbonitride does the rest.

Subject of Research: Effect of oxygen pressure during HVOF spraying on the structural-phase state, microstructure and tribological properties of Zr2CN coatings on tool steel

Article Title: Effect of oxygen pressure during HVOF spraying on the structural-phase state of Zr2CN coatings

Article References: Skakov, M., Kambarbekov, S., Uazyrkhanova, G., Kurbanbekov, S., Shektibayev, N., Baimbetova, G., Gradoboev, A., & Kaldar, B. (2026). Effect of oxygen pressure during HVOF spraying on the structural-phase state of Zr2CN coatings. Frontiers in Chemical Engineering, 8, Article 1842789. https://doi.org/10.3389/fceng.2026.1842789

Image Credits: AI Generated

DOI: 10.3389/fceng.2026.1842789

Keywords: HVOF spraying, zirconium carbonitride, Zr2CN, wear-resistant coatings, tool steel, oxygen pressure, X-ray diffraction, Rietveld refinement, tribology, microhardness, porosity, surface engineering