Surface treatment plays a critical role in enhancing the functionality of a component’s surface. Because different surface treatments operate through unique mechanisms, provide distinct functions, and have varied characteristics, their implementation timing also differs. Therefore, correctly selecting the appropriate type of surface treatment and rationally scheduling the treatment process are highly significant steps in ensuring that the treatment’s full potential and benefits are realized.
How to Choose Surface Treatment？
Choosing a surface treatment for a part is fundamentally about achieving a specific function or performance. To guarantee the best results, engineers should follow these core principles when selecting a treatment type.
Adaptability
The first step is determining if the chosen surface treatment is the most suitable for the part’s operating conditions and functional requirements. A thorough review of the component’s working environment is crucial.
Load Conditions
The magnitude of the load affects the potential for damage, which dictates the necessary thickness of the reinforcement layer.
For high loads that cause significant surface wear, methods that provide a thick, reinforced layer, such as thermal spraying, should be selected. The kind of force (e.g., impact, friction) determines the failure mode and influences the type of reinforcement needed.
Repeatedly alternating loads that cause fatigue damage, select reinforcement methods that produce a refined microstructure and minimal tensile stress, such as thermal spraying. Avoid thick plating processes, like hard chrome plating, in these cases.
Relative speed often affects the type of wear. If the damage is abrasive wear due to hard particles, consider methods like thermal spraying or hard chrome plating. If the damage is pure frictional wear, processes like zinc plating, copper plating, or nickel plating may be appropriate.
Environmental Conditions
The surrounding environment impacts material degradation.
If the medium contains hard particles, it will cause severe abrasive wear. Select treatments that provide a thick, high-hardness layer, such as hard chrome plating. For environments with severe corrosion that could lead to erosion corrosion or corrosion fatigue, choose materials with strong resistance. For seawater or chloride environments, use thermal spraying of austenitic stainless steel or duplex stainless steel, or copper plating. For acid/alkaline media, use austenitic stainless steel or nickel alloys. For SO2 containing media, consider aluminum materials. For general corrosion protection, zinc plating or black oxide may suffice.
Temperature Conditions
Extreme temperatures cannot be ignored, as they can cause failure modes like oxidation or thermal fatigue. Conditions that cause surface oxidation, thermal fatigue, or reduced thermal stability require materials like martensitic stainless steel or cobalt base alloys. Conditions that reduce material ductility and toughness, leading to brittleness, require materials like austenitic alloys or nickel base alloys.
Feasibility
This principle assesses whether the chosen surface treatment can be effectively and practically implemented on the specific component.
The component’s base material and its previous processing history must not conflict with the chosen treatment. For example, high-temperature processes like thermal spraying, where the surface is melted, are unsuitable for materials with low melting points.
If a treatment requires high temperature stress relief, but that temperature approaches or exceeds the tempering temperature of the base material, it will reduce the component’s hardness and strength. In this case, select a treatment that generates little or no residual stress and does not require high-temperature post finish.
Materials that are highly susceptible to hydrogen embrittlement (those with high hardness or strength) should not undergo processes that involve hydrogen evolution, such as pickling or certain plating methods, as this can cause cracking.
Complex parts with deep grooves, slots, holes, or sharp edges are not ideal for electroplating. Electroplating can result in uneven thickness and burrs in these areas, potentially causing the coating to peel. Thin-walled or easily deformed parts cannot withstand the high residual stress and heat of certain treatments. Low temperature, low residual stress treatments are necessary here.
Matching
The goal here is to match the service life of the treated surface to the expected life of the overall machine or assembly. The treatment should be durable but avoid “over-engineering.”
If a machine’s service life is 10 years, the surface treatment life should ideally be 10 years, or at least minimize the number of component replacements during that period. Treating a part for 15 years in a 10-year system is considered over-engineering and a waste of resources.
This concept helps quantify the effectiveness of a treatment.
ε = WH / WT
WH is wear on the component without surface treatment. WT is wear on the component with surface treatment. A higher e value indicates a better surface treatment effect.
A higher ε value indicates a better surface treatment effect. For example, if a part without treatment needs five replacements over 50 years, the ideal treatment should aim for ε equaling or greater than 5.
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Economic
The final consideration is achieving the required function at the lowest possible cost. A simple way to assess the economic viability is:
Cq ≤ Cw · （Sq / Sw）
Cq is the cost of the surface-finished component. Cw is the cost of the untreated component. Sq is the service life of the treated component. Sw is the service life of the untreated component.
Implementation of the Surface Treatment
Once the design engineer has determined the appropriate method, the manufacturing engineer must correctly implement the process to maximize its function.
The surface treatment must be scheduled correctly within the overall manufacturing steps.
Thick treatments like thermal spraying often cause significant deformation and require post-treatment stress relief and substantial machining. They should be scheduled after rough machining.
Treatments requiring surface grinding, such as hard chrome plating, should be scheduled after semi-finishing.
Processes that require minimal or no subsequent machining, like electroless plating, galvanizing, or high-energy beam treatments, should be scheduled after final finishing.
All surface treatments must occur after the component’s final bulk heat treatment. Performing the bulk heat treatment afterward will damage the surface treatment’s functionality.
Proper preprocessing of the surface is essential for strong bond strength between the treatment layer and the substrate. Most treatments require a smooth surface treatment, but some, like thermal spraying, specifically require surface roughening to enhance adhesion.
Some materials or treatments require stress relief before the surface process, while others do not.
A correct allowance is crucial, especially for thick or high-precision treatments. This allowance must account for the required thickness of the reinforcement layer and the subsequent machining/grinding needed. For components like nuts and bolts, an incorrect allowance on threaded areas can prevent the final product from meeting dimensional and tolerance requirements.
Post finish steps like stress relief or dehydrogenation are often required after certain processes and must be strictly scheduled and performed by the manufacturing team.
