Effect of sintering pressure on microstructure and mechanical properties of hot-pressed Ti6Al4V-ZrO2 materials
Graphical abstract
Introduction
Titanium and its alloys are very attractive materials for aerospace and chemical industries due their distinct mechanical properties such as high specific strength (strength to density ratio) [1], [2], [3], [4], corrosion resistance [1], [2], [3], [4], [5] and strength at relative high temperature [4], [6]. Additionally, since Branemark's discover these materials are widely used for biomedical applications such as orthopedic and dental applications due to their low weight [5], biocompatibility [2], [3], [4], [5] and ability to osseointegrate [4], [5] associated to their mechanical properties (toughness and fatigue strength) [7].
Regarding dental rehabilitation, the well documented beneficial results make titanium the gold standard material for the replacement of missing teeth [8]. However, one of drawbacks when using titanium and its alloys is the possibility of metallic ions release, causing hypersensitivity reactions [9].
Zirconia (ZrO2) is a very attractive ceramic for biomedical applications (such as dental implants and ball heads of femoral implant) due to its high resistance and flexural strength, wear resistance [10], thermal shock resistance [10], biocompatibility [11] and low affinity to bacterial colonization [12], [13].
In order to take advantage of both titanium and zirconia, a materials design with zirconia coatings have been used for coating titanium substrate to improve the tribological properties [14] and osteointegration [15]. However, due to shear stresses during implantation or operation, these coatings can be detached from metal surface and thus compromising the function of application. With the development of biomaterials science and industry technology, zirconia was proposed as a new material for hip head replacement and later for dental implants (due to its toothlike color) instead of titanium. However, ceramics are known to be sensitive to shear and tensile loading which imply a high risk of fracture [8].
In this context, the development of Ti6Al4V-ZrO2 bilayered materials produced by hot pressing seems to present an advantageous and original combination of materials and process for biomedical applications to surpass the disadvantages of the current solutions.
In the past decades, several studies have been conducted investigating the interfacial reaction between pure Ti and ZrO2 [16], [17], [18], [19], [20], [21], [22], [23]. Many of these studies performed at elevated temperatures indicated that distinct reaction layers can be formed at the interface. Furthermore, these studies indicated that oxygen-deficient and blackened ZrO2 was formed as result of oxygen dissolution into Ti. Lin and Lin [24] studied the diffusional reaction between Ti and ZrO2 at different temperatures (1100, 1300, 1400 and 1550 °C). They reported that interface is strongly dependent on temperature and that at T = 1100 °C the limited reaction resulted in t-ZrO2 − x. Lin and Lin [23] studied ZrO2-Ti composites produced by hot pressing, for thermal barrier graded materials, reporting that at higher sintering temperatures (1500 °C), Ti reacts with and is mutually soluble in ZrO2, resulting in the formation of oxides (such as α-Ti(O, Zr), Ti2ZrO and/or TiO) which is dependent on Ti/ZrO2 ratio. Teng and co-workers [25] describe that a phase transformation of zirconia (from tetragonal to monoclic) was found with increase of Ti volume fraction in the composites. Similar results were obtained by Lindong Teng and co-workers [26]. Furthermore, Correia et al. [18] studied the microstructure of diffusional zirconia-titanium and zirconia-Ti6Al4V alloy joints, reporting that at higher temperatures (T > 1300 °C), a weak oxide layer is developed from the direct ZrO2-Ti joints.
Another study conducted by Teng et al. [25] described that no reaction product was formed between Ti and ZrO2, and that interface bonding state is physical, for Ti-ZrO2 interfaces fabricated by hot pressing.
Even though extensive studies were carried out on the interface reactions, the pressure effect on the microstructure evolution (including blackening phenomenon on ZrO2) and mechanical properties (hardness and Young's modulus), at T = 1175 °C, for solid-state Ti6Al4V-ZrO2 materials produced by hot pressing, were not studied to date.
Regarding this processing technology, a hot-pressing method was used because it allows the manufacturing of near-net-shape products with near full densification, therefore eliminating finishing operations [6].
Section snippets
Materials
In this study, Ti6Al4V alloy spherical powder (TLS Technik GmbH & Co. Spezialpulver KG) with average particle diameter of 45 μm and commercial Yttria-stabilized zirconia (3Y-TZP) powder with uniform dispersion of 3 mol% Yttria (Tosoh Corporation) constituted by spherical granules (having an average diameter of 60 μm) of much smaller crystals that are about 40 nm in diameter. The chemical composition of titanium alloy and zirconia are presented in Table 1, Table 2, respectively. Scanning electron
Microstructural and chemical characterization
Fig. 4A), B) and C) show a schematic representation of the produced Ti6Al4V-ZrO2 bilayered materials revealing a blackening effect on zirconia layer with increasing pressure. Furthermore, a decrease of sample height with increasing sintering pressure can be observed, indicating a higher densification level for higher pressures.
Similar to conventional sintering processes, HP sintering is comprised by four stages: activation and rearrangement of particles, connection of particles, growth of
Conclusions
From the present study, the following conclusions can be drawn:
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This study confirms that hot pressing process could be used to produce fully dense Ti6Al4V-ZrO2 materials.
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Chemical composition and mechanical properties (hardness and Young's modulus) showed to be strongly dependent on sintering pressure. It was demonstrated that by increasing the sintering pressure, a reaction zone between Ti6Al4V alloy and ZrO2 as well as a color change on ZrO2 were found. Furthermore, an improvement in H and E is
Acknowledgements
This work is supported by FTC through the grant SFRH/BD/112280/2015 and the reference project UID/EEA/04436/2013, by FEDER funds through the COMPETE 2020 – Programa Operacional Competitividade e Internacionalização (POCI) with the reference project POCI-01-0145-FEDER-006941, and NORTE-01-0145-FEDER-000018-HAMaBICo.
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