Amorphous phase formation in plasma-sprayed hydroxyapatite coatings

Abstract

The amorphous phase content of air plasma-sprayed hydroxyapatite coatings is dependent upon spraying and deposition conditions. X-ray diffraction and optical microscopy were used to investigate the influence of spray parameters on the formation of the amorphous phase. Results show three factors which most influence the formation of the amorphous phase: dehydroxylation of the molten particle during flight, the cooling rate as it impinges onto the metal substrate, and the substrate temperature. Crystalline regions were identified as unmelted particles and elongated recrystallized areas. Amorphous phase regions vary throughout the coating but are more commonly found at the coating-substrate interface, i.e., the regions decrease toward the surface of the coating. Such an inhomogeneous distribution of phase content is expected to affect the clinical process of bone deposition, and therefore successful implant fixation.

Plasma-Sprayed Hydroxylapatite-Based Coatings: Chemical, Mechanical, Microstructural, and Biomedical Properties

“…Hence, the second shell heated to a temperature of 1360 < T < 1570°C, just below the incongruent melting point of HA, undergoes solid-state decomposition to a mixture of a 0 -TCP and TTCP. The third shell, heated to a temperature above 1570°C, consists of melt with a Ca/P ratio of 1.67 ( On impact with the target surface, the molten phase is rapidly quenched and will solidify quickly to produce ACP with various Ca/P ratios ( Ref 97,106,107). The force of impact of droplets accelerated to supersonic velocity triggers a series of events that profoundly affect the composition and the morphology of the resulting coating.…”

Effect of Heat Treatment on Characteristics of Plasma Sprayed Hydroxyapatite Coatings

“…21,44) Many studies claimed that the coatings adjacent to substrate have more amorphous phase. 8,35) When heattreated at higher temperature such as 700 C, the regions near the substrate may result in a larger amount of cracks caused by the large volume shrinkage during the crystallization and phase transformation, in turn, detrimental to the bonding strength of the coating, although possessing a higher crystallinity. Hence, in this work, the differences in the bonding strengths can be explained in terms of the stress relief and microstructure changes.…”

“…However, during plasma spraying process HA coating crystallinity decreases, phase composition of the coating is subjected to change, and residual stresses occur within the coating as a result of high temperatures. [7][8][9][10] Hence, plasma-sprayed HA-coated implants are essentially composed of a mixture of crystalline, amorphous, and non-apatite phases such as Ca 3 (PO 4 ) 2 (TCP), Ca 4 (PO 4 ) 2 O (TTCP) or even CaO. The presence of TCP and TTCP phases would enhance the resorption process of HA coating leading to implant instability.…”

Surface modification of titanium, titanium alloys, and related materials for biomedical applications

“…Abscissa is variable according to values or level of parameter. Amorphous content was determined by X-ray diffraction [239,240]. [244] and water vapor treatment [245].…”