Nitriding of Titanium and Titanium Alloys

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Introduction to Titanium and Its Properties

Titanium possesses remarkable properties that make it valuable for numerous engineering applications. With tensile strengths ranging from 200 to 1400 MPa (equivalent to most alloy steels) but only 56 percent the density of steel, titanium offers an exceptional strength-to-weight ratio. Its corrosion resistance rivals that of platinum, and it ranks as the ninth most abundant element in the earth's crust.

Despite these advantages, titanium and its alloys face significant limitations. Their poor resistance to oxidation restricts their operating temperatures to approximately 550°C, and they demonstrate inadequate wear resistance for many mechanical applications. These limitations have driven extensive research into surface modification techniques, with nitriding emerging as a particularly effective approach.

Fundamentals of Titanium Nitriding

Nitriding introduces nitrogen into the outermost surface of titanium components through a diffusion-controlled process. This surface modification creates a compound layer primarily composed of titanium nitrides (TiN and Ti₂N), followed by a nitrogen diffusion zone. The process time depends significantly on temperature, requiring engineers to carefully balance treatment speed against potential material distortion.

For most applications, optimal TiN layers range from 1-3 μm in thickness and exhibit surface hardnesses between 900-1100 HV0.05. Unlike Physical Vapor Deposition (PVD) treatments, diffusion-based nitriding eliminates the risk of layer delamination, as the layer grows from within the base material. Additionally, the diffusion zone (typically 20-40 μm thick) provides crucial support to the TiN layer.

While nitriding significantly improves the wear resistance of titanium alloys, particularly against adhesive wear, it does not enhance fatigue properties as it does with steel. In fact, excessively high nitriding temperatures can reduce the toughness of titanium alloys such as Ti6Al4V. Higher treatment temperatures may produce thicker layers more quickly but are only recommended for applications where wear improvement is the primary concern.

Pulsed Plasma Nitriding Technology

Conventional direct current (DC) plasma nitriding presents challenges when treating titanium alloys, particularly components with complex geometries. The high voltages and current densities required can cause overheating of thin sections, localized energy concentration, and potential arcing that damages surface finish. In deep slots or narrow openings, the "hollow cathode effect" can lead to workpiece melting, restricting treatment to simple geometries to avoid significant distortion.

Pulsed plasma nitriding addresses these limitations by decoupling the heating and surface treatment functions. Instead of maintaining a steady-state plasma, the system delivers spiked current and voltage pulses. This approach limits plasma energy input to only what is required for the desired metallurgical changes, with temperature control provided independently by resistance elements.

The microsecond-scale pulse duration reduces heat input significantly. Operating with duty cycles where the pulse is active for only 10-50% of the cycle does not affect nitrogen activity or nitriding time. The nature of the cycle can be adjusted during treatment to achieve microstructures with specific features, offering greater process flexibility and control.

Gas Nitriding Effects on Titanium Alloy Microstructure

The microstructure of gas-nitrided titanium alloys depends on both the chemical composition of the alloy and the processing parameters. Higher temperatures and longer treatment times promote grain growth and new phase formation on the material surface. During gas nitriding, nitrogen diffuses inward toward the metal substrate, forming a compound layer primarily consisting of titanium nitrides (Ti₂N and TiN). This is followed by a diffusion zone containing an interstitial solution of nitrogen in the α or β titanium phases. When nitrided below their β-transus temperatures, titanium alloys maintain a homogeneous microstructure. However, as the temperature increases beyond this threshold, the microstructure becomes inhomogeneous due to phase transformations occurring during the nitriding process.

Surface gas nitriding significantly increases the hardness of titanium alloys through the formation of titanium nitrides, which can reach hardness values exceeding 2,000 HV. The hardness gradually decreases with depth until reaching the core microhardness of the untreated material. Both the surface hardness and the thickness of the nitrided layer increase with higher temperatures and longer nitriding times.

The nitrided layer thickness, which can be estimated from microhardness profiles, typically ranges from 250 μm to 350 μm depending on processing parameters and alloy composition. For practical applications, nitriding at temperatures below the beta transus is recommended to maintain optimal material properties.

After nitriding, components develop a thin surface layer (5-20 μm thick) with an average microhardness of 1500 MPa. The nitrogen-enriched zone extends 0.1-0.15 mm into the material with microhardness values of 700-900 MPa. The optimal nitriding temperature range is 550-980°C; higher temperatures increase surface layer fragility, while lower temperatures dramatically reduce nitrogen diffusion rates.

Silicon Surface Alloying for Improved Oxidation Resistance

Poor high-temperature oxidation resistance remains a significant limitation for titanium alloys. Surface alloying with silicon has emerged as a promising approach to address this issue while also improving wear and creep resistance. Surface modification with silicon is preferred over bulk alloying because silicon significantly alters the mechanical properties of titanium.

Silicon enhances oxidation resistance through several mechanisms:

• It decreases oxygen penetration depth into the alloy substrate, consistent with its β-forming nature
• When dissolved in the TiO₂ surface layer, silicon reduces the diffusion rate of oxygen atoms
• It modifies stress relaxation processes in the oxide layer, contributing to the formation of a more compact layer with lower porosity

Several techniques are used for silicon surface modification of titanium, including laser surface alloying, silicon-ion implantation, vapor-phase siliconizing, and powder siliconizing.

Laser surface alloying involves rapid melting of a thin surface layer while simultaneously feeding silicon powder, resulting in a rapidly solidified Ti-Si alloy with an extremely fine microstructure. Silicon ion implantation can also improve oxidation resistance, though excessive ion doses or acceleration voltages may introduce lattice defects that enhance diffusion and diminish the benefits. Post-implantation annealing can partially mitigate these negative effects.

Despite their effectiveness, both laser surface alloying and ion implantation face limitations for widespread industrial application due to their high cost. Additionally, laser surface alloying often suffers from reliability and reproducibility issues.

Conclusion and Industrial Applications

Plasma surface treatment has become well-established as a flexible, cost-effective alternative to traditional salt bath and gaseous nitriding methods. The high level of flexibility and reproducibility achieved by pulsed plasma nitriding offers numerous opportunities for improving component performance and reducing life cycle costs across various industries.

For titanium alloys, nitriding provides significant improvements in wear resistance and surface hardness, though careful process control is essential to avoid compromising other mechanical properties. The selection of nitriding technique and process parameters should be tailored to specific application requirements, considering factors such as component geometry, service conditions, and performance priorities.