Materials Engineering

With its know-how, the Materials Engineering unit at Audi Quality Assurance supports material challenges across the entire process chain, from product creation to product testing.

Focused ion beam (FIB) – innovative analysis tool
In addition to conventional analysis techniques, such as light-optical microscopy, Audi’s materials experts also use scanning electron microscopes for analysis. These allow a significantly higher depth of analysis of the materials.

Scanning electron microscopy (SEM) provides high-resolution images of fracture surfaces or important insights into the concentration and distribution of chemical elements in the material being studied.

Materials Engineering has been using a focused ion beam (FIB) for four years now. This makes the brand with the four rings a pioneer among car manufacturers. The combination of an imaging scanning electron microscope and an ion gun as an abrasive tool makes it possible to create high-resolution, cross-sectional images of material systems.

The functional principle of the focused ion beam: the gallium ion beam of the FIB first digs a hole invisible to the human eye into the material under investigation. An integrated scanning electron microscope then provides a high-resolution, cross-sectional view below the material surface. The typical cutting depths of 5 to 50 micrometers – with widths of 50 to 100 micrometers – are achieved after two to five hours of cutting time. A major advantage is that the experts can follow the cutting process live with the scanning electron microscope.

At the same time, compared with mechanical preparation methods like the creation of grinding patterns, the FIB offers the big advantage in the case of complex material combinations (e.g. hard, thin coating of soft substrates) of only leaving behind minimal preparation artifacts. This refers to changes of a distorting nature, such as those on interfaces which result from such preparation.

In addition to the typical analyses in the field of surface technology, such as anti-corrosion coatings and paints, Audi uses FIB technology for almost all automotive-relevant raw materials and material systems: metal, glass, ceramics, polymers and even leather.

FIB analysis technology has particular potential for the new material challenges in electrification and digitalization. For example, a targeted and high-resolution SEM analysis of the individual layers of a touch-sensitive screen is only possible with an ion-based sample preparation. Possible weaknesses on interfaces of the individual layers which can lead to functional defects, can therefore be identified at an early stage in the development process.

CNG pressure tanks – analyses after shakedown testing
Audi customers can enjoy the A4 Avant g-tron (CNG consumption in kg/100 km: 4.3 - 3.8*; combined fuel consumption in l/100km: 6.5 - 5.5*; combined CO₂ emissions in g/km (CNG): 117 - 102*; combined CO₂ emissions in g/km (gasoline): 147 - 126*) and the A5 Sportback g-tron (CNG consumption in kg/100 km: 4.2 – 3.8*; combined fuel consumption in l/100 km: 6.3 – 5.6*; combined CO₂ emissions in g/km (CNG): 114 - 102*; combined CO₂ emissions in g/km (gasoline): 143 - 126*) as two sustainable drive variants. Both models are equipped with a bivalent 2.0 TFSI engine with a power output of 125 kW (170 hp). They can be powered, as with the A3 Sportback g-tron (CNG consumption in kg/100 km: 3.6 - 3.3*; combined fuel consumption in l/100 km: 5.5 - 5.1*; combined CO₂ emissions in g/km (CNG): 98 - 89*; combined CO₂ emissions in g/km (gasoline): 128 - 117*), either by means of climate-friendly Audi e-gas, normal CNG (Compressed Natural Gas) or regular gasoline. Their four high-strength, safe gas tanks are made from a mix of carbon-fiber reinforced polymer (CFRP) and glass-fiber reinforced polymer (GFRP). They are located underneath the rear of the vehicle. In order to achieve as high a range as possible when running on CNG (500 kilometers (310.7 mi)), the available installation space was optimally used. Each of the four pressurized tanks therefore has a different geometric shape to the next. Together, they store 19 kilograms (41.9 lb) of gas at a pressure of 200 bar.

The pressure tanks made of CFRP and GFRP fulfill the strict safety requirements prescribed by law. Before the start of series production, the materials engineering experts at Quality Assurance carried out elaborate analyses as part of shakedown testing. This ensures that the energy stores of the new g-tron models will function flawlessly even after many years and tens of thousands of kilometers in the hands of customers.

For the material experts, computed tomography is the non-destructive testing method of choice in a first analysis step. This technology allows, for example, fiber damage within the composite material to be made visible and measurable.

Microsections are prepared for further investigations of the pressure tanks. A section of the tank is cut out, one of its edges finely polished and then examined under the microscope. With this method, the tank’s composite material can be examined optically for pore size and distribution.

An additional thermal process (calcination) allows an exact determination of the porosity and a check of the proportion of glass fibers and carbon fibers. DSC analysis (differential scanning calorimetry) determines the softening temperature of the resin. This enables the experts to investigate the durability of the material after many thousands of kilometers.

In addition, the CNG tanks undergo further optical inspections and material analyses in the laboratory. One of these is tensile testing to determine the mechanical properties of the inliner, a special gas-tight blow-molded plastic part.

Sustainable: leather tanned using plant substances
Audi is a pioneer in the use of chrome-free tanned leather in the entire vehicle interior – and that since more than 20 years. Now follows the next step towards even more environmentally friendly use of resources: the brand with the four rings uses leather tanned using plant substances. This patented process uses olive leaf extract. The materials experts check and follow the diverse process steps from soaking to drying and staining, right up to the finished product – olive-leaf tanned leather.

* Figures depend on the tire/wheel set used and the transmission variant