3. Conclusion and Outlook
The results show, that the usage of yarn-based fabrics for CVD-stacking are not just increasing the fiber volume fractions and densities of W
f/W composites, but also increasing the reproducibility compared to the results presented in [
17] for 150 µm single fiber-based fabrics. In
Table 1, the material properties of each composite material produced is shown in an overview. The improvement can be explained with a significant increase of total number of fibers per volume unit (one yarn has 23 fibers with a total diameter of approx. 190 µm), an improved processability and a more homogeneous fiber distribution within the CVD-W matrix. This applies in particular for type 1, where the layer spacing is lower due to the reduced thickness of the weft material. Therefore, the usage of W-yarns as a warp material in combination with single weft fiber is a promising combination for the scale-up of W
f/W.
It should be emphasized that the application of an interface should further reduce the fiber-matrix adhesion and enhance the pseudo-ductile mechanisms such as the "pull-out" effect [
5,
6,
7,
25,
30,
31,
32,
33,
34]. The presented method to predict the fatigue behavior under cyclic mechanical loading tests might also pose an option to predict the lifetime under different stress conditions, such as thermal cyclic loading. However, the developed method needs to be further validated and investigated on further test samples.
Future efforts will focus on developing a more efficient manufacturing process that allows for the same or better material properties at significantly lower production costs- and times. Alternative production approaches such as the combination of the field assisted sintering technology with the CVD-process or the infiltration of stacked fabrics (CVI) are currently under investigation and will be presented in the future.
Figure 1.
Energy dissipation mechanisms in a fiber-based metal matrix composite: Pull-out of fibers, ductile deformation, crack bridging and crack wake- and front debonding of the applied ceramic oxide-based interface [
20,
25] .
Figure 1.
Energy dissipation mechanisms in a fiber-based metal matrix composite: Pull-out of fibers, ductile deformation, crack bridging and crack wake- and front debonding of the applied ceramic oxide-based interface [
20,
25] .
Figure 2.
Layer-by-layer CVD-process principle.
Figure 2.
Layer-by-layer CVD-process principle.
Figure 3.
Preform type based on single fibers - microscopic structure. Image was obtained with scanning electron microscope (SEM) Carl Zeiss LEODSM982.
Figure 3.
Preform type based on single fibers - microscopic structure. Image was obtained with scanning electron microscope (SEM) Carl Zeiss LEODSM982.
Figure 4.
CVD-stacking of 25 layers of single wire-based preforms [
17].
Figure 4.
CVD-stacking of 25 layers of single wire-based preforms [
17].
Figure 5.
Comparison: (a) Fabric type 1 with 50 µm weft filament; (b) Fabric type 2 with weft yarn. Top pictures show the tungsten fabrics as fabricated and were obtained via Zeiss optical microscope. Button images show metallographic cuts of each type and were made with a Nikon Eclipse LV 150 NL.
Figure 5.
Comparison: (a) Fabric type 1 with 50 µm weft filament; (b) Fabric type 2 with weft yarn. Top pictures show the tungsten fabrics as fabricated and were obtained via Zeiss optical microscope. Button images show metallographic cuts of each type and were made with a Nikon Eclipse LV 150 NL.
Figure 6.
CVD-process applied on three parallel coated fabrics.
Figure 6.
CVD-process applied on three parallel coated fabrics.
Figure 7.
Representative images of top- and sideview for both WfW composites after CVD-stacking (type 2).
Figure 7.
Representative images of top- and sideview for both WfW composites after CVD-stacking (type 2).
Figure 8.
Solid composites of six layers via CVD: Fabric 1 (a), Fabric 2 (b).
Figure 8.
Solid composites of six layers via CVD: Fabric 1 (a), Fabric 2 (b).
Figure 9.
Wf/W composite based on type 1. Image obtained with a Zeiss optical microscope.
Figure 9.
Wf/W composite based on type 1. Image obtained with a Zeiss optical microscope.
Figure 10.
Wf/W composite based on type 2. Image obtained with a Zeiss optical microscope.
Figure 10.
Wf/W composite based on type 2. Image obtained with a Zeiss optical microscope.
Figure 11.
TIRAtest 2820, Nr. R050/01 setup with KLST-type Wf/W samples for three-point bending tests and cyclic loading tests.
Figure 11.
TIRAtest 2820, Nr. R050/01 setup with KLST-type Wf/W samples for three-point bending tests and cyclic loading tests.
Figure 12.
Crack behaviour during three-point bending test from (a) unloaded state, (b) initial cracking (c) clear cracking and (d) almost full broken sample.
Figure 12.
Crack behaviour during three-point bending test from (a) unloaded state, (b) initial cracking (c) clear cracking and (d) almost full broken sample.
Figure 13.
Force over Displacement curves of KLST-type samples of fabric type 1, type 2 and sintered W-sample (Pure W powders 5 µm, field assisted sintering at 1900 °C, 50 MPa, 93 % rel. density).
Figure 13.
Force over Displacement curves of KLST-type samples of fabric type 1, type 2 and sintered W-sample (Pure W powders 5 µm, field assisted sintering at 1900 °C, 50 MPa, 93 % rel. density).
Figure 14.
Representative fracture images of type 2 for both composites, images taken with SEM Carl Zeiss LEO DSM 982. (a) illustrated crack bridging and (b) depicts ductile necking in the green- marked region and brittle fracture in the red-marked region.
Figure 14.
Representative fracture images of type 2 for both composites, images taken with SEM Carl Zeiss LEO DSM 982. (a) illustrated crack bridging and (b) depicts ductile necking in the green- marked region and brittle fracture in the red-marked region.
Figure 15.
Cyclic mechanical loading tests of composite type 1before (a) and after (b) 10,000 load cycles between 50-90 % of the rel. max. load capacity. The crack-growth is highlighted in yellow.
Figure 15.
Cyclic mechanical loading tests of composite type 1before (a) and after (b) 10,000 load cycles between 50-90 % of the rel. max. load capacity. The crack-growth is highlighted in yellow.
Figure 16.
Cyclic loading tests between 50-90 % of the avg. max. load - Representative overview for both composites.
Figure 16.
Cyclic loading tests between 50-90 % of the avg. max. load - Representative overview for both composites.
Figure 17.
Cyclic loading tests between 50-95 % of the avg. max. load of composite type 1.
Figure 17.
Cyclic loading tests between 50-95 % of the avg. max. load of composite type 1.
Figure 18.
Maxima of each sinusoid displacement [µm] over the total number of mechanical load cycles [-]. The red dots show each maximum, the blue line shows the trendline of a linear regression.
Figure 18.
Maxima of each sinusoid displacement [µm] over the total number of mechanical load cycles [-]. The red dots show each maximum, the blue line shows the trendline of a linear regression.
Figure 19.
Split function of the Displacement [µm] over total numbers of cycles [-]. The green marked area shows the values below 96 % of the critical displacement value as a linear trend, the red marked area describes the values above 96 % as an exponential growth.
Figure 19.
Split function of the Displacement [µm] over total numbers of cycles [-]. The green marked area shows the values below 96 % of the critical displacement value as a linear trend, the red marked area describes the values above 96 % as an exponential growth.
Table 1.
Comparison table.
Table 1.
Comparison table.