The high-temperature heat treatment of electroless nickel–phosphorus (Ni-P) coatings in an air environment, and its consequences have scarcely been investigated. This work investigated tribological characteristics of the high-temperature, heat-treated, electroless Ni-P coatings on steel substrates with low-, mid-, and high-phosphorus content for which the average phosphorus content was 2.4 wt.%, 7.1 wt.%, and 10.3 wt.%, respectively. X-ray fluorescence and energy dispersive spectroscopy were implemented to determine the phosphorus content of the coatings. The oxidation of Ni and the formation of the NiO layer on the coating surface was confirmed by the X-ray diffraction technique. A reciprocating sliding method on a ball-on-flat system was utilized to evaluate the coating’s friction and wear behavior. Among the coatings with varying phosphorus content, a high hardness of 1086 HV was found for high-phosphorus coating when heat-treated at 400 °C in an air environment, and that was decreased to 691 HV when heat-treated at 650 °C. The oxidation of nickel in the electroless Ni-P coating occurred when heat-treated at 400 °C in an air environment, and this phenomenon was increased more when the temperature was increased to 650 °C. The characteristics of the NiO layer that formed on the surface of the heat-treated electroless Ni-P coating were influenced by the concentration of phosphorus, which caused different colors of NiO to be seen on the Ni-P coating surface. A greenish black NiO layer on the low-phosphorus and black NiO layer on the mid- and high-phosphorus Ni-P coating was developed during heat treatment at 650 °C in an air atmosphere. The adhesion and tribological characteristics of the Ni-P coatings were affected by the NiO layer developed on the heat-treated Ni-P coating surfaces. The Ni-P coatings with mid- and high-phosphorus content showed enhanced wear-resistance characteristics when they underwent heat treatment in an air atmosphere at the high temperature of 650 °C. The wear volume obtained for as-plated mid-phosphorus and high-phosphorus Ni-P coatings was 0.111 mm³ and 0.128 mm³, respectively, and that was reduced to 0.031 mm³ and 0.051 mm³, respectively, after the high-temperature heat treatment.

Two different ceramic carbide nanoparticles (SiC, and TiC) were separately incorporated into the Ni–P matrix via the electroless deposition method. As prepared Ni–P, Ni–P–SiC, and Ni–P–TiC coatings were subjected to heat treatment at 400 °C for 1 h. The surface morphology, microstructural transformation, Vicker’s microhardness, tribological and scratch resistance properties were studied with reference to the different carbide reinforcements as well as heat treatment. Inter-nodular space, craters and kinks are created due to the branching effect of nodules in the surface of the Ni–P–SiC (TiC) composite coatings. After the heat treatment, the matrix phase transformation was not altered due to the incorporation of SiC or TiC into the Ni–P coating; however, a slight increase in residual stress was identified from the XRD analysis. In addition, the content of carbon deposition was found to be higher in the matrix of Ni–P–SiC composite coating than that in the Ni–P–TiC coating. The agglomeration of SiC particles was higher than TiC particles in the coating matrix, which was also supported by the result of Zeta potential measurement. Heat treatment improved wear and coefficient of friction in the Ni–P–SiC and Ni–P–TiC composite coatings. Compared to Ni–P–SiC coating, Ni–P–TiC coating revealed the enhanced tribological and scratch resistance performance after the heat treatment.

Nanomaterials are the subject of extensive investigations due to their applications in medicine, multimodal imaging, volumetric displays, and photonics. Here, lanthanide-doped bismuth vanadate (BiVO ₄ ) upconverting nanoparticles (UCNPs) have been reported. The nanoparticles have been synthesized by a microwave hydrothermal method. As-synthesized nanoparticles are highly crystalline in the tetragonal zircon phase with particles about 200 nm in size. Under 980 nm excitation, intense multicolor visible and near-infrared upconversion emissions are observed. Moreover, broadband infrared downshifting emissions are also observed. Time-resolved emission measurements have been carried out to investigate the involved upconversion and energy transfer mechanism. The BiVO ₄ -based UCNPs may provide a new class of nanomaterials for multifunctional applications.

Electroless Nickel–Phosphorus (Ni-P) coating is recognized mostly for its outstanding corrosion and wear-resistant behavior. The intrinsic corrosion and wear-resistant properties of Ni-P-based coating could be further upgraded by incorporating appropriate second-phase additive particles into the coating matrix. However, such properties of the Ni-P-based coating greatly rely on the surface and microstructural evolution arising with the co-deposition of the additive particles. In this study, submicron Si₃N₄ (average size ~200 nm) and nano Si₃N₄ (average size ~20 nm) particles were incorporated while depositing a Ni-P alloy in a low-carbon steel substrate to develop the Ni-P-Si₃N₄ composites through the electroless coating method. The 20 nm Si₃N₄-incorporated composite coating constituted fewer defects such as cavities and micropores on the surface, but such defects significantly appeared on the surface of the composite after the incorporation of 200 nm Si₃N₄ nanoparticles. Subsequently, the composite Ni-P-Si₃N₄, developed with the co-deposition of 20 nm nanoparticles, is enriched with enhanced anticorrosion characteristics compared with the composite developed with 200 nm nanoparticles. The enhancement of anticorrosion behavior was attributed mainly to the Si₃N₄ nanoparticles that covered the substantial volume of the coating and led to inhibit the formation of corrosion active sites such as defects and metallic Ni phase.