Patrick Faulkner, lead engineer at Accu, looks at how surface roughness can affect component performance and discusses the implications for manufacturing processes and quality control in the aerospace industry.
It is widely recognised that even the most minute details can have profound implications in aerospace engineering. One such factor is surface roughness in components and structures.
For aerospace engineers, understanding and optimising surface roughness is crucial for ensuring the safety, efficiency and longevity of aircraft and spacecraft. From the fuselage to engine components, every part must meet exacting standards to withstand the extreme conditions encountered during flight.
Understanding surface roughness
Surface roughness refers to the fine irregularities present on the surface of a material. These microscopic peaks and valleys, often invisible to the naked eye, have a significant impact on how a component interacts with its environment and other parts.
In aerospace applications, where components are subjected to extreme stresses and environmental conditions, the nature of these surface irregularities can mean the difference between success and failure.
Surface roughness is typically measured using parameters such as Ra (average roughness), Rq (root mean square roughness), and Rz (average maximum height). Each of these measurements provides different insights into the surface characteristics.
Understanding these parameters is crucial when specifying surface requirements for different components. For example, a turbine blade and its associated fixings may require a different surface roughness profile compared to a fuselage panel, based on their respective functions and operating conditions.
The impact of surface irregularities
Surface roughness can influence the interaction between a component and its environment, especially in aerospace. At the microscopic level, these surface irregularities can result in friction and wear. It is widely known that smoother surfaces generally result in lower friction, which is vital for moving parts in engines and control systems.
Surface roughness also affects fluid dynamics – as the surface profile can significantly impact airflow, affecting drag and heat transfer.
It also impacts adhesion properties, as it plays a role in how well coatings, paints and adhesives bond to a component.
Finally, it affects fatigue resistance, as the presence of surface irregularities can act as stress concentrators, potentially initiating cracks under cyclic loading.
The aerospace industry is renowned for its wide use of aluminium and titanium, due to their properties of being strong and lightweight. However, surface finish, particularly surface roughness, plays a pivotal role in determining the fatigue behaviour, thermal integrity and aerodynamic efficiency of aerospace components.
This is especially true for parts manufactured using advanced techniques, such as Laser Powder Bed Fusion (LPBF).
The importance of surface finish extends beyond purely aesthetics; it directly impacts stress concentration, crack propagation, residual stress and contact fatigue.
Impact on aerodynamic performance
One of the most significant impacts of surface roughness is its effect on aerodynamic performance. The aerospace industry’s constant pursuit of fuel efficiency and improved flight characteristics make the control of surface roughness a top priority. For example, on the Spitfire, moving from Domed Rivets to flush rivets led to an improvement of almost 20mph in flight.
The boundary layer, a thin layer of air close to the surface of an aircraft, is highly sensitive to surface roughness. A smoother surface can promote a laminar boundary layer, which results in less drag. However, increased roughness can cause a premature transition to turbulent flow, leading to increased skin friction drag, higher fuel consumption and reduced range and endurance.
In high-speed flight regimes, even minor surface imperfections on components can lead to performance penalties.
For aluminium and titanium components maintaining an optimal surface finish is therefore crucial to ensure that aerodynamic efficiency of the aircraft is optimised.
Surface roughness doesn’t just affect drag; it also influences lift generation. The precise contours of wing surfaces, for example, are designed to create specific pressure distributions. Any deviation from the intended surface profile, due to excessive roughness, can alter pressure distributions, which potentially affects stall characteristics, lift coefficient and overall flight stability.
Aerodynamic noise
Another often-overlooked aspect of surface roughness is its impact on noise generation. Rougher surfaces can increase turbulence in the boundary layer, leading to higher levels of aerodynamic noise and increased cabin disturbance. This can also potentially cause issues with meeting community noise regulations.
For both commercial and military aircraft, managing noise through careful control of surface roughness is becoming increasingly important, especially in components exposed to high-speed airflow.
How does temperature affect surface roughness?
The high-temperature environments experienced in aerospace mean the relationship between surface roughness and thermal management becomes critical. This is particularly relevant for components made from aluminium and titanium, which are often used in areas exposed to significant heat loads.
The surface roughness of aerospace components can significantly affect their heat transfer characteristics. For example, rougher surfaces generally have less contact area, potentially reducing heat conduction between mating parts. Surface roughness also influences the boundary layer behaviour, affecting convective heat transfer rates. In addition, the emissivity of a surface, which affects radiative heat transfer, is influenced by its roughness.
For heat exchangers and engine components made from aerospace-grade aluminium and titanium, optimising surface roughness is crucial for maintaining efficient thermal management systems.
Many aerospace components, especially those in hot sections of engines, rely on thermal barrier coatings (TBCs) for protection. The effectiveness of these coatings is closely linked to the surface roughness of the underlying material. A controlled level of surface roughness is often necessary to ensure proper adhesion of TBCs, rougher surfaces can mean less affective adhesion.
In addition, when surface roughness and high temperatures are combined it can exacerbate fatigue issues, which is particularly relevant for engine components.
In extreme cases, excessively rough surfaces can lead to uneven coating thickness, potentially compromising thermal protection.
Stress concentrations
Surface roughness can create localised stress concentrations, which are often the initiation points for fatigue cracks. Even small surface irregularities can act as notches, amplifying local stresses.
These stress concentrations provide favourable sites for crack nucleation under cyclic loading.
The surface texture can influence the path of crack propagation, affecting overall fatigue life. As aerospace components are often subjected to high cyclic loads, minimising these stress concentration effects through proper surface finishing is crucial.
For aerospace components, optimising surface roughness is crucial for maintaining long-term fatigue resistance. Rougher surfaces can trap corrosive agents, which can accelerate corrosion-assisted fatigue.
In components with mating surfaces, such as riveted joints common in aircraft structures, surface roughness can also influence fretting behaviour and subsequent fatigue life.
How surface roughness influences corrosion
The increased surface area in the cases of rougher surfaces provides more opportunities for corrosion initiation. Surface irregularities can create micro-crevices, which are prone to localised corrosion.
For materials like aluminium and titanium that rely on passive oxide layers for corrosion protection, surface roughness can affect the uniformity and stability of these protective layers. Optimising surface finish is therefore crucial in minimising potential corrosion initiation sites for aerospace components.
Reducing surface roughness
There are a number of post manufacturing surface treatments which can achieve final surface roughness specifications. These include etching and passivation, which alter surface characteristics of aluminium and titanium components.
Techniques like shot peening and polishing are used to modify surface roughness and induce beneficial residual stresses. In addition, anodising for aluminium and electropolishing for titanium can enhance surface properties while controlling roughness.
Electrical conductivity
Whilst not a primary consideration, the relationship between surface roughness and electrical conductivity is important in certain aerospace applications, particularly for components made from conductive materials like aluminium. This becomes crucial in applications ranging from electromagnetic shielding to lightning protection systems.
Future trends and innovations
As aerospace technology continues to advance, new approaches to managing and optimising surface roughness are emerging. These innovations promise to enhance the performance, durability and efficiency of aerospace components, particularly those made from materials, such as aluminium and titanium.
Emerging technologies are enabling the development of surfaces that can adapt to changing conditions, with shape memory alloys which can alter their surface characteristics in response to temperature or stress, potentially offering dynamic control of surface roughness.
Innovative self-healing surfaces are being developed, with materials that can repair surface damage autonomously, which could help maintain optimal surface roughness over time.
Ultra-thin coatings with precisely controlled surface structures can offer enhanced properties while maintaining tight dimensional tolerances.
New manufacturing and finishing methods are being trialled to achieve unprecedented control over surface roughness. In laser surface texturing, high-precision laser systems can create tailored surface textures for specific applications, such as improved lubrication or reduced drag.
Refined Electrochemical machining techniques offer the potential for creating ultra-smooth surfaces on complex geometries, which is particularly beneficial for aluminium and titanium components in aerospace applications. These advanced techniques are enabling aerospace manufacturers to achieve surface finishes that were previously impossible or impractical.
The integration of AI and machine learning is set to revolutionise surface roughness control in aerospace manufacturing with predictive modelling and defect detection and classification becoming ever more important.
These AI-driven approaches are set to enhance the consistency and quality of surface finishes in aerospace components, while potentially reducing manufacturing costs and lead times.
To summarise, the importance of surface roughness in aerospace applications cannot be overstated. From its impact on aerodynamic performance and thermal management to its role in fatigue life and corrosion resistance, surface finish is a critical factor in the design and manufacture of aerospace components.
As the aerospace industry continues to push the boundaries of performance and efficiency, particularly with the use of lightweight materials like aluminium and titanium, the control and optimisation of surface roughness will remain an important area. Emerging technologies and innovations in component protection are helping to meet the demanding requirements of modern aviation and space exploration.