Because of the excellent strength-to-weight ratio and corrosion resistance, titanium alloys are widely used in aerospace, electronic devices and other fields. However, it is easy to form a dense passivation film on the surface, which seriously restricts the stability of the plating process. In this article, the current core methods for improving the binding force of titanium alloy matrix and coating are systematically reviewed, which provides the theoretical reference for the engineering practice.
Pretreatment and strengthening technology
Surface activation modification method
The mechanical sandblasting pretreatment can achieve the dual effect of passivation film stripping and surface coarsening by the high-speed impact of 60-120 mesh emery. The experimental data show that the bond strength of TA2 pure titanium specimen after sand blasting can reach 3.2 times of that of untreated specimen. However, it should be noted that the high-strength titanium alloy > HRC40 is prone to stress concentration, and it is necessary to control the sandblasting pressure < 0.4MPa.
Hydrogenated film generation: Using HCl(500ml/L)+TiCl3(15ml/L)+ corrosion inhibitor system, treated at 40℃ for 5min, can form a thickness of about 200nm TiH₂transition layer. XPS analysis showed that the layer formed Ti-TiH2 eutectic structure with the substrate, and the binding energy increased to 28MPa.
Fluorinated membrane modification: NaCr2O7(250g/L)+HF(20ml/L) mixture was treated at room temperature for 30s to form a TiF3 /TiO₂composite membrane. SEM observation shows that the film layer has a honeycomb structure, which can effectively improve the anchoring effect of the coating.
Metal transition layer deposition
- Gradient zinc leaching process
Secondary zinc plating was adopted: primary (ZnSO₄480g/L, HF 120ml/L, 25s) → Deplating (HNO₃50%) → secondary zinc coating was obtained (coverage > 98%). The practice of an electronics factory in Nanjing showed that the bonding force of copper coating was increased from 3.5N/mm² to 15.6N/mm² by this process.
- Electroless nickel plating as a base
By NaH₂PO₂(30g/L)+NiSO₄(25g/L)+ complexing agent system. 2μm Ni-P layer was deposited at 85℃. The Electron microscope analysis showed that Ni-Ti intermetallic compounds were formed between the coating and the substrate, and the shear strength reached 45MPa.
Enhanced treatment technology after plating
1. Vacuum heat treatment: under the vacuum degree of 10^-3Pa, 300℃×2h treatment can make the Cu/Ti interface diffusion layer thickness of 1.5μm, and the bond strength is increased by 40%. It is necessary to pay attention to the β phase transition temperature (pure titanium 882℃) to avoid matrix phase transition.
2. Pulse current annealing: the use of 20kHz high frequency pulse, treated at 200℃ for 30min, can promote the directional diffusion of coating atoms. An application of an aerospace component showed an increase in the binding strength of the gold coating from Class 4B to the highest class ASTM D3359 .
Process selection strategy
1. Recommended for precision electronic components: chemical nickel-plating + pulse annealing (size deformation < 0.1%)
2. Suitable for structural parts: sandblasting + hydrogenated film + high temperature diffusion (30% cost reduction)
3. Suggestions for special environmental components: Fluorinated film + flash nickel plating (corrosion resistance increased by 5 times)
The current technical breakthrough direction focuses on atomic layer deposition (ALD) nanotransition layer and laser assisted plating technology, which is expected to increase the bond strength to 200MPa. In engineering practice, it is necessary to design personalized process routes according to matrix type (α/β titanium alloy), coating functional requirements (conductive/wear-resistant) and cost constraints.