Improving the surface characteristics of Ti-6Al-4V and Timetal 834 using PIRAC nitriding treatments

doi:10.1016/j.surfcoat.2018.01.051

Surface and Coatings Technology

Volume 339, 15 April 2018, Pages 208-223

https://doi.org/10.1016/j.surfcoat.2018.01.051 Get rights and content

Highlights

•
The effect of temperature and alloy on the PIRAC nitriding response for Ti has been investigated.
•
Ti-6Al-4V forms a thick compound layer consisting of Ti2N and TiN.
•
Timetal 834 only forms TiN and an underlying nitrogen enriched zone.
•
Compound layers were well adhered to the substrates for both alloys.
•
Both alloys had improved surface hardness and tribological behaviour.

Abstract

Despite the popularity of a number of techniques of thermochemical diffusion for titanium, in many cases the surface engineering processes used may not be economically viable options for industry. This work focuses on the application of Powder Immersion Reaction Assisted Coating (PIRAC), a relatively inexpensive nitriding treatment that can provide a remarkable improvement in the surface characteristics of titanium alloys. The aim of this work was to determine whether PIRAC could be successfully applied to Ti-6Al-4V and the high-performance near-α titanium alloy Timetal 834. In order to study the response of these materials to PIRAC nitriding, techniques such as X-ray diffraction, micro-indentation hardness, surface profilometry, optical and electron microscopy, nano-scratch adhesion testing and ball-on-plate reciprocating-sliding wear testing were employed. These techniques highlighted the markedly different response between the two alloys to the PIRAC treatment; namely, that Ti-6Al-4V forms a thick compound layer, while at the same processing temperature and time Timetal834 does not form any appreciable Ti₂N phase instead forming a nitrogen-diffusion case with a thin TiN compound layer at the surface. This inherent difference in nitridability influences the metallurgical response of each alloy. Despite this, the surfaces of both alloys were still hardened considerably and their tribological performance in dry sliding conditions improved compared to the untreated alloys.

Section snippets

Background and introduction

Titanium alloys are used in various industrial sectors due their advantageous properties such as a high strength-to-weight ratio, excellent corrosion resistance and a maximum service temperature of up to 600 °C [1]. This generally explains their popularity in sectors such as the aerospace, chemical, medical and sports industries. However, titanium alloys suffer from poor tribological performance, which often limits their use to static structural applications. Without the use of a protective

Substrate materials and PIRAC process

Cylindrical mill-annealed Ti64 and alpha-beta heat-treated Ti834 coupons of 25 mm diameter and 5 mm and 3 mm thickness respectively were prepared. The average surface roughness, R_a, of the coupons prior to treatment was 0.01 ± 0.005 μm. The chemical composition for both as-received substrates as observed using Energy Dispersive X-Ray Spectroscopy (EDX) is shown in Table 1.

Both alloys were PIRAC nitrided at 700 °C, 800 °C and 900 °C for a duration of 2 h using the treatment methodology as

Surface characterisation

After PIRAC nitriding, the substrate microstructure of both Ti alloys, shown in Fig. 1, becomes visible and the surfaces gain a golden hue, which varies slightly depending on the processing temperature used. The treated coupons show no noticeable edge effects. In the case of the Ti64 alloy, typical nitride ‘islands’ cover the sample surface of the parent alloy while in the case of the Ti834 alloy the substrate microstructure remains clearly visible in all PIRAC nitrided samples. Generally, for

Conclusions

The principal aim of this work was to assess and compare the ability of PIRAC nitriding to improve the tribological performance of Ti64 and Ti834 alloys. Furthermore, the effects on the surface characteristics of the two different alloys following PIRAC processing were studied.

Ti64 and Ti834 respond in markedly different ways to the PIRAC nitriding treatment. Following PIRAC processing Ti64 formed a thick compound layer, especially evident at higher processing temperatures, consisting of Ti₂N

Acknowledgments

The authors gratefully acknowledge ERDF (Malta) for the financing of testing equipment employed throughout this project: “Developing an Interdisciplinary Material Testing and Rapid Prototyping R&D Facility (Ref.no. 012)” and the scholarship funding made available throughout the Strategic Educational Pathways Scholarship Scheme (Malta).

The authors also wish to thank the technical staff at DMME laboratories, especially Ing. James Camilleri, Mr. Noel Tonna and Mr. Daniel Dimech for their

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Enhanced strength and sliding wear properties of gas nitrided Ti-6Al-4V alloy by ultrasonic shot peening pretreatment
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A novel ultrasonic shot peening (USP) was performed on Ti-6Al-4V alloy as a pretreatment prior to gas nitriding (GN). The samples treated by USP/GN composite processes were systematically investigated in terms of microstructure, strength and sliding wear properties. The results indicated that a surface nitriding layer composed of TiN, Ti₂N and α-Ti(N) phases with a thickness of 20–30 μm was formed on the samples after USP/GN composite process treatment. The USP pretreatment could significantly improve the microhardness and thickness of nitriding layer, and the enhancement is positively correlated with the intensity of USP Almen. Compared with conventional GN without pretreatment, the maximum microhardness (∼810HV_0.025) and thickness of nitriding layer were increased by 30.43 % and 26.92 % by USP/GN composite treatment with Almen intensity of 0.356 mmA, respectively. Meanwhile, the wear resistance was significantly enhanced, and the width of abrasive mark was reduced by 51 %, compared with that without pretreatment. This work offers a potential approach to improve the nitriding process of Titanium alloys.
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View full text

Improving the surface characteristics of Ti-6Al-4V and Timetal 834 using PIRAC nitriding treatments

Highlights

Abstract

Section snippets

Background and introduction

Substrate materials and PIRAC process

Surface characterisation

Conclusions

Acknowledgments

Mater. Sci. Eng. A

Surf. Coat. Technol.

Surf. Coat. Technol.

Surf. Coat. Technol.

Surf. Coat. Technol.

Surf. Coat. Technol.

Mater. Sci. Eng. A

Vacuum

Surf. Coat. Technol.

Mater. Charact.

Mater. Sci. Eng. A

Mater. Lett.

J. Eur. Ceram. Soc.

Int. J. Refract. Met. Hard Mater.

Surf. Coat. Technol.

Appl. Surf. Sci.

Surf. Coat. Technol.

Wear

Wear

Thin Solid Films

Acta Mater.

Acta Mater.