Strategies to improve the corrosion resistance of microarc oxidation (MAO) coated magnesium alloys for degradable implants: Prospects and challenges

The development of composite/nanocomposite coatings on various surfaces based on electroless process has much interest among researchers due to their enhanced mechanical, electrical, magnetic, and optical properties compared with their metallic or alloy counterparts. These coatings have shown many applications in automotive, aerospace and industrial fields with unique characteristics obtained by the incorporation of secondary particles as reinforcement at micro or nanoscale in the matrix. However, properties of the product depend upon not only the nature of secondary particle used but also the specific characteristics of the metal matrix. This paper critically reviews the recent developments in micro and nanostructured metallic composite coatings formed by electroless coating techniques and retrospect into the various mechanisms of the electroless coating process, its preparation with different metallic systems, properties, and applications in detail.

Metallic composite coatings by electroless technique – a critical review

In present study, the structural and wear properties of as-plated and heat-treated electroless Ni-P alloy and Ni-P-Si 3 N 4 composite coatings on iron based powder metal (PM) compacts were investigated. The coatings were prepared using hypophosphite reduced alkaline electroless nickel bath. The synthesized submicron α-Si 3 N 4 particles (10 g/l) were added into the Ni-P bath to develop Ni-P-Si 3 N 4 composite coatings on the PM compacts. Coated compacts were heat-treated at 300, 400 and 500 °C, for 1 h and the effect of heat treatment on the coating performance was studied. Chemical composition of the coatings was analyzed by EDX. The phases and morphology of Si 3 N 4 particles and coatings were examined by XRD and SEM. Phase transformation behavior was studied by differential scanning calorimetry (DSC). The wear properties were studied using pin-on-disc tribometer under unlubricated sliding conditions under a normal load of 40 N, sliding speed of 1 ms − 1 and sliding distance of 1000. The results show that the incorporation of submicron Si 3 N 4 particles in the deposit significantly has been found to improve the hardness and wear resistance of composite coatings. The hardness and wear resistance of Ni-P and Ni-P-Si 3 N 4 coatings heat-treated at 400 °C had the maximum hardness and wear resistance.

Structural and wear properties of heat-treated electroless Ni-P alloy and Ni-P-Si3N4 composite coatings on iron based PM compacts

Electroless Nanocomposite Coatings: Synthesis, Characteristics, and Applications

Ni–B alloy deposits were successfully formed over alkali surface treated boron carbide by electroless technique and the effects of processing parameters such as reduction bath pH, temperature and post-heat treatment were investigated. Microstructural studies show remarkable changes in structural morphology from mesh-like structure to platelet type Ni–B coating. The main Ni–B phases observed were NiB, Ni2B, Ni3B and Ni4B3. An effective Ni–B alloy coating with better surface characteristics was formed at pH 8 under 75°C reaction bath temperature in the presence of bath stabiliser. The post-heat treatment of Ni–B coated particles under varying bath reaction temperatures at 400°C showed remarkable Ni–B phase transformation from metastable NiB and Ni2B to stable crystalline Ni3B phase. The NiB coated particle shows lower particle–matrix interfacial reaction, better wettability and enhanced dispersion in aluminium composites compared to uncoated particles.

Studies on electroless nickel boride coating on boron carbide particles

The idea of codepositing various second phase particles in electrodeposited (ED) or electroless deposited (EL) metal or alloy matrix and thereby taking advantage of their desirable qualities such as hardness, wear and abrasion resistance, corrosion resistance, etc., has led to the development of ED and EL composite coatings with a wide range of possible combinations and properties. This chapter presents an overview of the deposition methodologies involved in preparing ED and EL composite coatings, the mechanism of codeposition of particles, challenges and issues on particle incorporation in the metal matrix, particularly on the dispersibility of particles and the interaction between particle and metal matrix during deposition. In addition, the influence of particle incorporation on the nucleation, plating rate, adhesion, surface appearance, roughness, microstructure, texture, hardness, tribological characteristics and corrosion resistance of composite coatings and some of the key applications of composite coatings are addressed.

T. S. N. Sankara Narayanan

Papers

Electro - and electroless deposited composite coatings: preparation, characteristics and applications