Will surface treatment weaken the strength of the parts?

一, The main purpose of surface treatment is to strengthen and toughen at the same time.
Surface treatment is not just one technology; its main purpose is to improve performance by modifying the way materials' surfaces are structured and stressed. There are two main types of surface treatment based on how they work:
1. Improved treatment: makes the surface harder and more resistant to wear and tear
Shot peening strengthening: This method uses high-speed projectiles to hit the surface and create a residual compressive stress layer that is up to 0.5mm thick. This can enhance fatigue strength by more than 200%. For instance, shot peening can make the fatigue life of aviation engine blades last longer than 10 ^ 7 cycles of load, going from 500 hours to 1500 hours.
Laser shock peening: A high-energy laser creates plasma shock waves that create a 1mm-deep layer of residual compressive stress on the surface. This makes the grain size smaller, which makes titanium alloy parts three times more resistant to fatigue.
Carburizing/nitriding: A chemical heat treatment creates a very hard carbide or nitride layer on the surface (up to 1200HV), which makes the surface far more resistant to wear. After carburizing, the hardness of the surface of automotive gears went from 35HRC to 60HRC, and the life of the gears was extended by five times.
2. Toughening treatment: slows down the spread of cracks
Surface rolling: By rolling a roller over the surface, processing imperfections are removed and residual compressive stress is created. This slows the rate at which cracks spread in aluminium alloy parts by 60%.
Phase transformation toughening: For materials like zirconia ceramics, sandblasting causes the surface to change from t phase to m phase. The compressive stress from volume expansion is then used to fight the force that causes cracks to spread, which makes the bending strength go up by 15% to 20%.
Key conclusion: Scientifically designed surface treatment can make parts much stronger instead of weaker by using methods like residual compressive stress, grain refinement, and phase transformation toughening.
二, The danger of bad craftsmanship: the key point between improving strength and making performance worse
Surface treatment can make things stronger, but if the process parameters aren't regulated or the materials don't work well together, the strength may actually go down. This is mainly due to the following three mechanisms:
1. Too much hardening makes things break easily.
A company used too much temperature carburizing treatment on stainless steel valves to make them more resistant to wear. This made the carbide layer on the surface thicker than 0.8mm, and the carbides built up at the grain boundaries, which caused cracks and made the valve fail early in pressure testing.
Mechanism: When the surface hardness is higher than the core material's toughness limit, cracks are likely to spread from the hard, brittle layer to the soft core. This is called a "hard and brittle" failure mode.
2. Residual tensile stress speeds up the start of cracks.
Case: Improper electroplating treatment caused residual tensile stress to build up at the contact between the coating and the substrate of a certain car gearbox shaft. The crack density rose by three times when the sample was subjected to alternating stress.
Mechanism: If electroplating, chemical plating, and other processes don't keep the coating's stress state in check, tensile stress may be added to balance off the strengthening effect of surface compressive stress.
3. Damage to the surface causes stress to build up.
After being sandblasted at high pressure, microcracks appeared on the surface of zirconia ceramic implants. In simulated chewing tests, the crack propagation rate was twice as rapid as that of untreated samples. This meant that the danger of early fracture in clinical use was much higher.
Mechanism: If the settings for mechanical treatments like sandblasting and grinding are wrong (for example, if the pressure is too high or the abrasive particles are too small), the surface can be damaged deeper than the compressive stress layer, which can cause a fracture to start.
The main point is that the negative effect of surface treatment on strength is caused by bad processing, not the technique itself. To eliminate risks, you should optimize parameters and test quality.
三, Material properties and process adaptability: the main idea behind strength optimization
The physical attributes of different materials, like how hard or tough they are and how they change phases, directly affect how you choose and set up surface treatment techniques. The following are common ways to modify materials:
1. Metal materials: balancing of residual compressive stress and hardness
Titanium alloy: Shot peening (with a diameter of 0.6mm and a pressure of 0.4MPa) is the first step to avoid scratching the surface with harsh abrasives like silicon carbide. After processing, acid washing is needed to get rid of any abrasives that are stuck in the surface.
Aluminium alloy: To create residual compressive stress without making the surface too rough or lowering its fatigue strength, glass bead sandblasting (with a particle size of 120 mesh and a pressure of 0.3MPa) is used in combination with anodizing.
Stainless steel: Using low-temperature nitriding (520 °C) and stainless steel shot blasting (particle size 80 mesh, pressure 0.5 MPa) to balance surface hardness and corrosion resistance.
2. Ceramic materials: toughening through phase change and damage control
Zirconia ceramics: The pressure of the sandblasting should be less than 0.25MPa and the time should be less than 20 seconds. This will keep the surface damage depth from being greater than the thickness of the compressive stress layer (approximately 50 μm). Alternatively, laser etching with a low energy density (≤ 5J/cm²) can be used to prevent thermal cracking.
Silicon nitride ceramics: To make a microporous structure, chemical etching (HF+HNO3 mixed acid) is the best method. To improve adhesive strength without causing mechanical damage, mechanical locking is utilized.
3. Composite materials: strengthening the contact and stopping delamination
Plasma spraying (5kW power, 30L/min argon flow rate) is used to make a metal transition layer on the surface of carbon fibre reinforced composite material. This makes the coating stick better and prevents fibres from breaking when they are directly sandblasted.
Laser cladding (power 2kW, scanning speed 10mm/s) deposits wear-resistant coatings on the surface of metal-based composite materials. The heat input is carefully managed to keep the substrate and reinforcing phase from separating.
The main point is that the qualities of the material dictate how adaptable the process is, and the "Material Process Performance" database should be used to guide parameter design. For instance, the "Surface Treatment Process Specification" (GJB 5098-2008) set the process window for different materials in the aviation area.