Hello,
After a little more investigation (I'm not familiar with KS), KS indeed use .mtl to store materials description, but it's some sort of a KS .mtl version, as the data structure is quite different from what a wavefront .mtl.

KS code :

#define material "William Wallace" {
surface "lux_general" {
"diffuse" 0.639216 0.639216 0.639216,
"specular" 0 0 0,
"ambient" 0 0 0,
"alpha" 1,
"reflectivity" 0,
"indirect_multiplier" 1,
"texture_modulate" on,
"shadow_modulate" off,
"texture" xX433a2f55736572732fd0a0d0b8d0ba2f446f63756d656e74732f4b657953686f742f54657874757265732f706c6169643031522e6a7067,
"texture_shift_u" 0,
"texture_shift_v" 0,
"texture_tile_u" 2,
"texture_tile_v" 2,
"texture_wrapmode_u" 1,
"texture_wrapmode_v" 1,
"texture_angle" 0,
"normalmap" "plaid01L.jpg",
"normalmap_shift_u" 0,
"normalmap_shift_v" 0,
"normalmap_tile_u" 2,
"normalmap_tile_v" 2,
"normalmap_wrapmode_u" 1,
"normalmap_wrapmode_v" 1,
"normalmap_angle" 0,
"normalmap_height" 1,
"specular_shift_u" 0,
"specular_shift_v" 0,
"specular_tile_u" 1,
"specular_tile_v" 1,
"specular_wrapmode_u" 1,
"specular_wrapmode_v" 1,
"specular_angle" 0,
}
}



Wavefront .mtl

newmtl V1
Ns 19.999998
Ni 1.500000
d 1.000000
Tr 0.000000
Tf 1.000000 1.000000 1.000000
illum 2
Ka 0.588000 0.588000 0.588000
Kd 0.588000 0.588000 0.588000
Ks 0.378000 0.378000 0.378000
Ke 0.000000 0.000000 0.000000
map_Ka Z:\LGrass\PaQGrass\Textures\BladeColorV1.jpg
map_Kd Z:\LGrass\PaQGrass\Textures\BladeColorV1.jpg


In other words KS .mtl only works in KS, and it's unlikely you will find any application that could write this kind of .mtl.


PBR in this context means Physical Based Rendering.
Long story is here : https://www.marmoset.co/posts/physically-based-rendering-and-you-can-too/

It's not a file format, but more a 'better' way to describe material attributes. And somehow it's easier to transfer PBR material (textures maps) between different render engines.
So if you design some materials in Substance, you have to rebuild the shader in KS, using for exmaple this guide : (as Stephan already posted)

https://support.allegorithmic.com/documentation/display/SPDOC/Keyshot