Modification of properties (AQA GCSE Design and Technology): Revision Notes
Modification of properties
Materials in electronic and mechanical systems often need their properties changed to make them more suitable for specific applications. This process is called modification of properties, and it involves various techniques to improve characteristics like strength, durability, and resistance to wear.
Hardening and tempering of metals
Metals that experience heavy wear, such as components in gears and bearings, can be made stronger through a process called hardening. This technique works particularly well with carbon steel.
The hardening process involves moving carbon atoms within the steel's structure to create a new arrangement that makes the material harder but also more brittle. The carbon steel is heated to its critical temperature of 900°C, then rapidly cooled by plunging it into water. This quick cooling process is called quenching, and it locks the carbon atoms in a hard but brittle structure.
This extreme hardness often makes the metal too brittle for practical use, which is why tempering is essential to prevent catastrophic failure in real-world applications.
However, this is where tempering comes in. Tempering reduces the brittleness while maintaining much of the increased hardness and improving the material's ability to resist impact.
Worked Example: Tempering Process
Step 1: Clean the hardened steel thoroughly with emery cloth until you can see the natural oxide colours on the surface
Step 2: Heat the steel to a specific temperature (for example, 295°C), which you can judge by the colour changes in the steel
Step 3: Quench the steel again in water to lock in the new properties
Case hardening
Gears and cams in mechanical systems experience significant surface wear during operation. Case hardening provides an excellent solution by increasing the carbon content only on the outer surface of steel components, leaving the core tough and resilient.
This process is particularly valuable because it creates components that have the best of both worlds - a hard, wear-resistant surface combined with a tough, impact-resistant core that won't crack under stress.
Worked Example: Case Hardening Process
Step 1: Heat the steel component to approximately 950°C, which appears as a cherry red colour
Step 2: Place the heated component in a carbon-rich bath containing case hardening compound or powdered carbon
Step 3: Repeat this process several times until the desired amount of carbon has been absorbed into the outer surface of the steel
Each cycle adds more carbon to the surface layer, gradually building up a hard, wear-resistant exterior while preserving the toughness of the inner core.
Anodising aluminium
Aluminium makes an excellent material for electronic components because it doesn't rust. However, it does oxidise naturally, which creates a dull finish over time. Anodising solves this problem by creating a controlled, decorative, and durable oxide finish that's also highly resistant to corrosion.
The term "anodising" comes from the fact that the aluminium component acts as the anode (positive electrode) in the electrochemical process, where it attracts negatively charged oxygen ions.
Worked Example: 'Hardcoat' Anodising Process
Step 1: The aluminium component is thoroughly cleaned to remove any contaminants
Step 2: The aluminium part (which acts as the anode) is immersed in an acid electrolyte bath alongside a cathode made of lead or aluminium
Step 3: An electrical current flows through the setup, travelling from the anode to the cathode
Step 4: This current causes oxygen atoms to be released from the acid electrolyte, which then react with the aluminium surface to create a thick oxide layer
Step 5: Finally, dye can be added to colour the finish, and the surface is sealed to prevent fading
This process creates a finish that's not only attractive but also provides excellent protection against corrosion and wear.
Making photo-resist printed circuit boards
Printed circuit boards form the backbone of electronic systems, and their layouts can be designed using computer software and then manufactured using photo-resist techniques. The photo-resist method uses light-sensitive materials to create precise circuit patterns.
Photo-resist technology is essential in modern electronics manufacturing because it allows for extremely precise circuit patterns that would be impossible to create by hand, enabling the miniaturisation we see in today's electronic devices.
Worked Example: Complete PCB Manufacturing Process
Step 1: Cutting and preparation - Start with a photo-resist board and cut it to the required size, then carefully peel off the protective layer to expose the sensitive surface
Step 2: Mask placement - Place the photo-resist mask (containing your circuit design) onto the sensitive side of the PCB, ensuring perfect alignment
Step 3: UV exposure - Expose the board to UV light for a specific length of time. The light passes through the clear areas of the mask but is blocked by the dark areas, creating a pattern in the photo-resist material
Step 4: Development - Develop the exposed board using dilute sodium carbonate for 15-30 seconds, then rinse thoroughly. This removes the photo-resist from the exposed areas
Step 5: Etching - Use ferric chloride solution to etch away the unwanted copper for 15-45 minutes, then rinse the board clean
The result is a precisely manufactured circuit board with the exact pattern needed for your electronic system.
Key Points to Remember:
- Hardening strengthens metals by heating to 900°C and rapid cooling, but tempering is needed to reduce brittleness
- Case hardening creates tough components with hard surfaces by repeatedly heating steel in carbon-rich compounds
- Anodising aluminium produces decorative, corrosion-resistant finishes through controlled electrochemical oxidation
- Photo-resist PCB manufacturing uses UV light to create precise circuit patterns through a 5-step process
- Temperature control is critical in all these processes - specific temperatures and times must be followed for successful results