Mechanical characteristics of anodized coatings
In the automotive industry weight reduction – and the associated fuel savings – are top priority, which is why lightweight materials such as aluminum are used. In order to withstand mechanical stresses, however, these softer components must be made wear resistant. For this reason, hardcoat (Type III) anodization is becoming ever more common.
While hard anodized coatings are typically 30-80 µm thick, some are only a few µm! For these coatings, conventional hardness measurement systems that rely on optical evaluation of the indentation (e.g. Vickers method) approach the limits of their ability. A much better suited method is the instrumented indentation test, which can be applied to measure not only the hardness in terms of plastic deformation (HV), but also to assess other quality-determining characteristics. Using the instrumented indentation test, even very thin anodized coatings can be analyzed without risking influence from the substrate.
For such technical applications hard anodized coatings must have a consistent hardness of 400-600 HV across the entire section. Soft anodized coatings for decorative applications have a hardness of about 200-400 HV, which is reached a few hundred nm below the surface.
The FISCHERSCOPE® HM2000 with its ESP (Enhanced Stiffness Procedure) mode is able to determine mechanical properties like the Vickers hardness or the elastic indentation modulus dependent upon the depth.
Figure 1a/b shows the Vickers Hardness HV (calculated from the indentation hardness HIT) and the indentation modulus EIT of two coatings: a hard anodized coating (480 HV) of 11 µm thickness (shown in red) and a soft anodized coating of 14 µm thickness (shown in blue). The higher standard deviation for the hard anodized coating stems from the roughness of its surface.
Fig.1a: derived data for Vickers hardness (HV) of a hard anodized (red) and a soft anodized (blue) coating
Fig.1b: indentation modulus (EIT) of hard anodized (red) and a soft anodized (blue) coating
In Figure 1a one clearly sees the consistent hardness of the hard anodized coating and the increasing hardness of the softer anodized coating, which also exhibits less elasticity (Figure 1b, indentation modulus). On the hard anodized coating, the elasticity decreases as one approaches the substrate.
The FISCHERSCOPE® HM2000 is optimally suited for the precise determination of the mechanical characteristics of thin anodized coatings. Beside the hardness, other parameters such as the plastic or elastic material characteristics can be accurately assessed. Please contact your local FISCHER representative for further information.
Nanoindentation on wear-resistant DLC coatings applied to engine components
In order to reduce emissions in combustion engines without sacrificing performance, manufacturers are continually working to improve the ability of the moving parts (e.g. camshafts, piston rings and gears) to resist abrasion and reduce friction. Coating them with DLC (diamond-like carbon) is just such an optimization. DLC coatings are not only very hard but also feature a certain toughness – which are two of the critical parameters that must be monitored during the coating process.
Composed primarily of amorphous diamond and amorphous graphite, DLC coatings serve first and foremost as protection against wear and tear but they also minimize friction. Due to their dark colour and the miniscule size of the indentation, determining their hardness by optically measuring the indenter impression is almost impossible and therefore unreliable.
A more accurate method for testing DLC coatings is nanoindentation, during which the force and displacement are continuously measured during both the loading and unloading phases. From these data, one can calculate the hardness and other quality-determining characteristics, such as the modulus of indentation. This method also prevents the substrate material from exerting any influence on the measurement results.
In this example, the measurement results of a 3 µm thick DLC layer are presented, as determined using the FISCHERSCOPE® HM2000. The Martens hardness (HM) takes the plastic and elastic deformation of the sample into account. The modulus of indentation (EIT), however, also allows conclusions to be drawn regarding the elastic behaviour. The values for penetration hardness (HIT) and the resultant converted Vickers hardness (HV) indicate the plastic properties of the samples.
Fig.1: Martens hardness (HM) and other parameters of the DLC coating. The table shows mean value, standard deviation und coefficient of variation of 12 measurements; the graph shows the depth-dependent profile.
The standard deviations and coefficients of variation illustrate the accuracy with which these quality-related parameters can be determined, even on rough samples with thin coatings. But the FISCHERSCOPE® HM2000 also makes it simple to take these highly sophisticated measurements:
- Extremely fast sample preparation
- Short measuring times
- High depth resolution
- Minimal, thus negligible, device compliance
When it is utterly crucial to determine the mechanical properties of DLC coatings with speed, accuracy and precision, the FISCHERSCOPE® HM2000 is indispensable. For further details, please contact your local FISCHER representative anytime.
Measuring Nikasil® coatings on aluminum automotive cylinders
Automotive cylinders are subject to extreme mechanical wear. A way of protecting aluminum cylinders is to plate the contact surfaces with Nikasil®, a hard coating technology used to improve the tribological characteristics of the cylinder parts and to optimize the heat transfer. It is often used for high-end engine components of premium car brands and in motor sports.
Nikasil® (from the German for Nickel-Carbide-Silicon – Nickel-Karbid-Silizium) is an electroplated nickel matrix with embedded silicon-carbide particles. Its amazing tribological characteristics make Nikasil® the ideal contact surface for pistons and rings; it is therefore used on high-end engine parts to minimize the friction within the engine block. The Nikasil® coating is appropriate for two- and four-stroke aluminum cylinder walls or sleeves and is used in a variety of automotive engines ranging from classic cars to the latest Formula 1 vehicles.
Fig. 1: Cylinder block made of aluminum coated with Nikasil®. Coating thickness approx. 20 - 50 µm
In practical use the cylinder walls or sleeves are electroplated with a layer of Nikasil®, typically between 80 and 180 µm thick. Afterwards, the coating thickness is controlled using the custom-configured table-top FISCHERSCOPE® MMS® PC2, further outfitted with a NICKELSCOPE® module and the probe ENW3. Overall, this specialized instrument employs the magnetic measurement method (hall effect).
In the next step the cylinder barrels are milled and honed to improve their sliding characteristics. A final coating thickness somewhere in the range of 25 to 50 µm is aimed for, which requires extremely narrow tolerances to be met in the measurement procedure.
Fig. 2: FISCHERSCOPE® MMS® PC2
The probe ENW3 is the centerpiece of the measurement system and is used both after the electroplating process as well as after honing. Its angled form helps the user to reach even difficult-to-access places.
Measuring inside cylinder barrels or sleeves can also be automated. To this end, the FISCHERSCOPE® MMS® PC2 can be equipped with a digital I/O module to control up to six probes at once; measurement values are collected and transferred to the instrument simultaneously, making it possible to take multiple measurements with high frequency for quality control purposes in a running production line.
For precise thickness measurement of Nikasil® coatings the FISCHERSCOPE® MMS® PC2 – equipped with the NICKELSCOPE® module and the ENW3 probe – is the perfect choice for professional quality monitoring of Nikasil® coatings. Even automated measurements with multiple probes are feasible. Please contact your FISCHER representative for more information.
Measurement of thick NiP coatings on automotive parts
In the automotive industry, the plungers used inside the solenoid valves of automatic transmission gearboxes must fit smoothly into their through-holes to an accuracy of just a few µm, in order to prevent oscillations that would lead to jamming or canting. To meet these tight tolerance limits, the plungers must be coated very evenly, which requires strict quality control.
In the manufacture of parts in the automotive and machine-building industries, adhering to extremely tight tolerance limits is necessary to guarantee the components’ proper functioning. That is why electroless metal platings like electroless nickel are being used more and more frequently, as they enable a very even coating: The layer builds up more homogeneously and with less variation in thickness than electroplated coatings, which tend toward excessive coating thicknesses on edges and corners.
In this example, steel plungers for solenoid valves are coated with approximately 60-70 µm of NiP containing at least 10% phosphorous. Afterwards, the parts are ground to an accurate fit; the end thickness of the coating is approximately 50 µm, which must be within a tolerance range of ± 4 µm. This layer is itself non-magnetic and can, for purposes of incoming inspection and/or after grinding, be measured with the magnetic induction method using the DUALSCOPE® FMP100 and the FGAB 1.3 probe.
Ø 3 µm *
Ø 0.3 µm *
Control of the measurement system variation by repeated measurements on a single measurement spot
Tab.1: Measurement results of a quality inspection
* 10 readings taken on different measurement spots per sample
The DUALSCOPE® FMP100 and FGAB1.3 probe are employed in conjunction with a V12 BASE stand, which makes it possible to replicate the measurement procedure with consistent probe positioning and angle. This minimizes operator influence and produces extremely repeatable results, as shown in Table 1: The standard deviation for the measurements of the coating after grinding is, on average, just 0.3 µm, and the variation of the entire measurement system itself is only 0.03 µm, which is negligible. Therefore the measurement device capability even for the required tight tolerances is fulfilled.
Fig.1: DUALSCOPE® FMP100
The DUALSCOPE® FMP100, together with the probe FGAB1.3 and the stand V12 BASE, forms a reliable control system that can precisely and accurately measure NiP coatings on automotive components with minimal variation. This allows both monitoring of quality specifications and adherence to very tight tolerance limits – and therefore, the avoidance of potentially costly warranty claims. For further information please contact your local FISCHER representative.
Fischer Instruments K.K.
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